most of the times you require to open the mdl in notepad and check if everything has coherence.
First of all look for parsing errors, normally when the mdl is convertable, there are none of these.
Second, make sure that everything be coherent, vertexes must be attached to existing bones, bones must relate the geosets that asre jointed to them, if its only one geoset, the bone must reference it, as well as its geosets anims. something very common is that bones point to multiple geosets and are assigned to a geosetanim which is a mistake. You should also look that if your model uses geosets anims, each geoset has a geoset anim.
Third look for extraordinary values, like negative values in color data fields of particles, lights, etc.