Did you switch engine providers? FYI, IIRC if you switch engine providers before a restock, you do NOT get a new complement of 20 engines - you are stuck with the same amount of engines you had with the previous provider.
That's not exactly true, at least for me. In my 1st season in iGP, I switched engine providers a few times, and still had a restock after 20 races.
What Jake said is correct I believe.