Need some help for android issue


Hello, sorry, the question is not according to the book, but I can’t find answer or advice and decide to ask same here.
The problem is: I have several tv-boxes, working via wi-fi in different city points - show video ads. To check they are show video correctly, I decide to take screenshots with some interval and compare them. Ok, I take screenshot by this command:
adb shell screencap -p /sdcard/screen.png && adb pull /sdcard/screen.png
sometimes I get good screen shot:

but other time (> 95%) I get just alpha-blank screenshot -
All videos are play throught media player
`private MediaPlayer mMediaPlayer;
Android 4.2.2. Any ideas ?