i’ve got the exact same problem with the pitft 2.8 capacitive.
did everything correct according to the guide on the adafruit site but the screen stays white,
hdmi says screen size not suported as it is 320×240.

tried to doe the source instalation of retropie but even that didnt work.

/etc/init.d/asplashscreen

#! /bin/sh
### BEGIN INIT INFO
# Provides:          asplashscreen
# Required-Start:
# Required-Stop:
# Default-Start:     S
# Default-Stop:
# Short-Description: Show custom splashscreen
# Description:       Show custom splashscreen
### END INIT INFO

do_start () {

    /usr/local/bin/fbcp &
    line=$(head -n 1 /etc/splashscreen.list)
    isMovie=$(echo $line | grep -o "*.mpg")
    if [ -z "$isMovie" ]; then
      /usr/bin/fbi -T 2 -once -t 20 -noverbose -a -l /etc/splashscreen.list &
    else
      mplayer $line &
    fi
    exit 0
}

case "$1" in
  start|"")
    do_start
    ;;
  restart|reload|force-reload)
    echo "Error: argument '$1' not supported" >&2
    exit 3
    ;;
  stop)
    # No-op
    ;;
  status)
    exit 0
    ;;
  *)
    echo "Usage: asplashscreen [start|stop]" >&2
    exit 3
    ;;
esac

:

/boot.config.txt

# uncomment to force a HDMI mode rather than DVI. This can make audio work in
# DMT (computer monitor) modes
#hdmi_drive=2

# uncomment to increase signal to HDMI, if you have interference, blanking, or
# no display
#config_hdmi_boost=4

# uncomment for composite PAL
#sdtv_mode=2

# for more options see http://elinux.org/RPi_config.txt
gpu_mem_256=128
gpu_mem_512=256
gpu_mem_1024=384

overscan_scale=1
dtparam=spi=on

# --- added by adafruit-pitft-helper Sun May 10 21:46:15 UTC 2015 ---
[pi1]
device_tree=bcm2708-rpi-b-plus.dtb
[pi2]
device_tree=bcm2709-rpi-2-b.dtb
[all]
dtparam=spi=on
dtparam=i2c1=on
# uncomment if you get no picture on HDMI for a default "safe" mode
#hdmi_safe=1

# uncomment this if your display has a black border of unused pixels visible
# and your display can output without overscan
disable_overscan=1

# uncomment the following to adjust overscan. Use positive numbers if console
# goes off screen, and negative if there is too much border
#overscan_left=16
#overscan_right=16
#overscan_top=16
#overscan_bottom=16

# uncomment to force a console size. By default it will be display's size minus
# overscan.
#framebuffer_width=1280
#framebuffer_height=720

# uncomment if hdmi display is not detected and composite is being output
hdmi_force_hotplug=1

# uncomment to force a specific HDMI mode (this will force VGA)
hdmi_cvt=320 240 60 1 0 0 0
hdmi_group=2
hdmi_mode=87

# uncomment to force a HDMI mode rather than DVI. This can make audio work in
# DMT (computer monitor) modes
#hdmi_drive=2

# uncomment to increase signal to HDMI, if you have interference, blanking, or
# no display
#config_hdmi_boost=4

# uncomment for composite PAL
#sdtv_mode=2

# for more options see http://elinux.org/RPi_config.txt
gpu_mem_256=128
gpu_mem_512=256
gpu_mem_1024=384

overscan_scale=1
dtparam=spi=on

# --- added by adafruit-pitft-helper Sun May 10 21:46:15 UTC 2015 ---
[pi1]
device_tree=bcm2708-rpi-b-plus.dtb
[pi2]
device_tree=bcm2709-rpi-2-b.dtb
[all]
dtparam=spi=on
dtparam=i2c1=on
dtparam=i2c_arm=on
dtoverlay=pitft28c,rotate=90,speed=64000000,fps=60
# --- end adafruit-pitft-helper Sun May 10 21:46:15 UTC 2015 ---

pi@retropie ~ $ tail /var/log/messages
May 11 08:15:53 retropie kernel: [   35.406377] Bluetooth: BNEP socket layer initialized
May 11 08:15:53 retropie kernel: [   35.443387] Bluetooth: RFCOMM TTY layer initialized
May 11 08:15:53 retropie kernel: [   35.443468] Bluetooth: RFCOMM socket layer initialized
May 11 08:15:53 retropie kernel: [   35.443528] Bluetooth: RFCOMM ver 1.11
May 11 08:15:54 retropie fbcp[2461]: Primary display is 320 x 240

i think fbcp doesnt run properly in my case.
if i use tail /var/log/messages only the primary display is detected and not the secondary display,

ampersands what does tail /var/log/messages say with your pi?

started off again with retropie 2.6 didn’t work, started off with 3.0b2 still white screen.
Looks like fbcp isn’t running because the secondary display isn’t showing.
Tried again with both versions and did apt-get update and upgrade but still white screen.
Tried the resistive drivers instead of capacitive but again the screen stays white.
started off again with the pitft image and then installed retropie white screen again.

Installing with hdmi or ssh makes no difference.
Hdmi is working and retropie boots perfectly at 320×240 Screen resolution.

I Did update and upgrade after the first boot and controler setup and checked afterwards if retropie was working correctly before I installed the pitft helper.

maybe it’s a PAL/NTSC problem?
Not sure if hdmi uses PAL And if fbcp doesn’t work with PAL.

use dtoverlay parameter to pitft28-resistive on new kernel version
and check file /boot/overlays/pitft28-resistive-overlay.dtb

dtoverlay=pitft28-resistive,rotate=90,speed=80000000,fps=60

Try to test it.

Reference
https://github.com/raspberrypi/firmware/tree/master/boot/overlays