I assume you mean they tap on "OK" on the traveller and don't know how to get back to the traveller screen in order to tap on the Hand button. If there are more boards to play in the current round, it's possible to go back to an earlier traveller in the same round by tapping on the board number when the next traveller is displayed. This will display a set of board numbers for the current round - tap on one of these to display the traveller for an earlier board, and then the Hand button.
If play has progressed to the next round the hand can still be input at the end of the session. On the Ranking screen there is a "Scorecard" button (assuming that you are using the latest BridgePal Server release - 2.0.5 at the time of writing). The Scorecard display shows the scorecard for NS or EW. Tapping on a board number in the left hand column of this display takes you to the traveller for that board, from where you can input the hand.
Another point is that hands can be input at any table that is currently playing them, it does not have to be during Round 1. So, if Table 2 omits to enter the hands during Round 1, Table 1 can enter those hands during Round 2 (assuming boards move downwards), though they may grumble at the extra work !
I wouldn't like to make it mandatory to enter the hands before advancing to the next screen. This would not go down well at bridge clubs where players have run out of time in the current round.
John