LOOKUP won't count blank spaces

The development of my department tracking system (attached) has hit a snag and I would appreciate any ideas you may have, please, to resolve the problem.

Assessment data is added as shown - 4A, 5C, etc, into columns N, O , P, Q, R and S. I am using a lookup table to convert that into a numerical value, with the intention of adding up the total average value, shown in column T. The problem is that, if there are blank cells within the range, it seems unable to add them up. For the first student, row 2, that is fine. For the other students, because their assessments are ongoing, hence there are blank cells, I am unable to get a running score.

Hope all this makes sense.