Domestic (home) sewing machines are not designed to sew leather that is thick, dense, or large, or requires thread larger than B69 (T70).
It doesn't matter what brand name is on the body. If it has drop feed only, and neither a roller or walking foot drive, it is not going to feed leather properly. The result of trying to sew leather that is beyond the handling/feeding capacity of a machine is one or more of the following.
Skipped stitches
Shorter than usual stitches, or stitches going into same hole and not moving along
Top thread knots on bottom, or bunched up on bottom, or wrapped around bobbin
Bent and broken needles
Bent take-up lever or cranks inside head
Shuttle hook knocked out of time
Needle bar knocked up, out of time with hook.
Ruined leather goods