Summary fields don't recalculate with related values

[lk]

I hope this is the right place to post this ...

Field A calculates a value based upon a related file. It will not update unless I click on the field and then, only for that record.

Field B is a summary field totaling field A. It will not update even when field A's value changes as above.

If I do something like redefine field B, then it updates, but not by changing field A values.

Any help will be greatly appreciated.

Thanks.

[comment]

What is the calculation used by Field A?