There are a number of reasons why transactions may not come through when making a GetIncrementalInserts call on Donation record type. It could be synchronization window, if the window does not coincide with when the transactions were made then the GetIncrementalInserts call will not return anything.

There is also a possibility that the form in question is a designated form for a TeamRaiser Event which renders all the gifts made through this form "TeamRaiser transactions". TeamRaiser transactions are not stored in the Donation record type but TeamRaiser record types namely TeamRaiserGift and TeamRaiserRegistration.