If you were to do the identical exam questions on your own in exam conditions, but with say 25% extra time, how much better would you do? How about with 50% extra time? How about with notes? How about with a calculator? Have you practiced with similar questions in homework and in exam prep?
It's hard to identify exactly what the problem is. Please also clarify how you know you have a great understanding. Tests are about demonstrating your understanding so there are many links on this chain.