Numeric Entry Fields

Hello Everyone

I'm in the middle of a project that has numeric entry fields.

The learner can input a grade and then can continue. The grade is sent to a matrix for an overview of scores.
The user also has the option of going back and changing the grade they gave based on some feedback. Problem starts when they try this.
The numeric entry field won't allow it. It seems locked and nothing can be inputted again.
Is there any way this can be changed to accept a change?

Thanks

Frazer