The M1 Carbine was designed to reliably feed ball, soft point didn't figure into the design.Some feed them without problems, many will even feed the CorBon DPX. First would be trying a different magazine. Second thought is the mag catch. If you have been using 30 rounds mags without changing to an M2 mag release it can cause the mag to sit slightly lower. Ball will still bounce in but the exposed lead on a soft point may catch. Changing mag catches may solve if that is the problem.
Last might be the ramp, polishing lightly may help.