When the iPad goes to sleep (also known as auto-lock) while in Capture Duo or Capture for Ipad is in record mode, it will stop drawing the waveform. Recording still occurs but you will not see a waveform from the point your iPad goes to sleep. You will be able to playback your recording in it's entirety and you can also transfer your session to Studio One and you will see the full waveform drawn.
To prevent your iPad from going to sleep on it's own you can change the delay time for this feature to turn on.
To change this,
1) Go into your iPad Settings,
2) Select General on the left column
3) In the right column scroll down and select Auto-Lock.
4) There you can choose the time duration for how long the iPad will auto-lock / go to sleep. The default is 2 minutes.
NOTE: if you set it to Never, your iPad battery will dissipate very quickly, to avoid this consider adjusting the level of brightness to extend battery life.
To adjust iPad brightness
1) Go into your iPad Settings
2) Select Wallpapers & Brightness (located directly below General) on the left column
3) On the Brightness page, there is a slider, adjust it to a setting as low as you're comfortable viewing your screen with.