How do you post your payments? (I.e. Escrow, Interest, Principle, then Late Charges?) You should still be collecting for escrow up front, even though they’ve changed their payment amount. (Especially if this escrow account is mandatory on their loan due to being considered an HPML.) One way or the other, something’s going to end up short.
Like JGo said, I’d definitely contact the customer and inquire why they’re not sending in the additional payments.
You might want to do a preliminary analysis to figure out where they stand if you haven’t been collecting the escrow payment.