I have a 550B, but the instructions for the Square Deal B say you push the lever forward to seat primers, so that much is the same. Shell plate center bolt and spring-loaded detent ball look the same. Primer slide appears to be similar.
If primers don't seat completely, you can look at -
- shellplate is loose, allowing case to tilt as primer is inserted
- primer slide isn't adjusted correctly
- lever isn't being pushed all the way forward
I have never bothered to clean primer pockets, but I have read reports of primer residue causing trouble with seating primers. But that would only apply if the cases had been reloaded many, many times and not cleaned.