Troubleshooting suggestions.
1. Is the long groove of the needle facing away from the shuttle? (Required for the loop to form)
2. If you manually feed the shuttle thread up the hole in the needleplate, and thread the top thread through the needle as normal, is the effort required to pull them roughly the same?
3. If you remove the front slide plate and manually advance the machine by turning the handwheel the correct direction, do you see a loop being formed?
4. If you see a loop in step 3, do you see your shuttle trying to go in the loop?
The answers to these will be a big help