sometimes that happens.
Try to set your radio to the SAME frequency again, before you create a logbook entry later on.
The other station will not stay on the same freq. if you try it later - and that point can create a mistake in the system.
So it is very important to use the SAME time (of QSO), that the other operator wrote on his card. Maybe +/- a couple of minutes.
Than type a value for every orther field: name, callsign (of course) and RST. Save the entry (maybe twice, if an error-message appears). Should work now.
73 and good DX from Jörg :-)
Bochum / Germany