Check in the assessment model if gearing factor exists for technology
BFP = LOC / gearing factor
There is a value for gearing Factor, specific to each technology which is used to calculate the Backfired IFPUG.
So if this value is not associated in the assessment model then BFP will not be calculated.
You can check the same using the page - Functional Weight review [and adaptation] - Backfired Function Points
Note that these are the technologies for which the gearing factor value is already added by default in the assessment model - CMS Assessment model - Information - Functional weight - Backfired IFPUG Function Point Definition
If your technology is Microsoft T-SQL then this is a known bug that is planned to be fixed in 8.1.5
The solution is to add the technology filter for "Microsoft T-Sql" with a gearing factor of value 60 in assessment model for Backfired IFPUG to be calculated.