Opportunity Revenue calculation

Hello,

On our opportunities we use:

Revenue: System Calculated
Est. Monthly Premium
Est. Annual Premium (this calculates based on the monthly).

However, we've had a few users notice that the Est. Annual Premium isn't always calculating, and one one particular example that I checked it showed a Pricing Error and Missing Price Level in the Audit. We aren't using pricing levels so I'm wondering why this would be a factor.

Our system is defaulted currently to System Calculated and it usually works, however occasionally it doesn't and we have to change Revenue to User Provided in order to enter an Est. Annual Premium. 

Any recommendations would be greatly appreciated!

Thank you,

Brianna