Math Field doesnt recalculate when fields are populated
The (fairly new) Math Field module for D7 is a not commonly known/used alternative to Computed Field). 

However, there are still a few other (known) issues with it, as mentioned also on its project page:


The cTools math expression library causes an error when using function that take more than one parameter. This effects pow(), min(), max() (#1958538: Improve math expression engine).
Math Field does not yet support multivalue options fields (checkboxes, or multiselect) (#2483453: Add aggregate functions for multivalue fields).


However, if you're trying to use this module to handle multivalue fields, then that would explain why you can't get that to work (as per the issue mentioned before: multi value fields are not yet supported).