Developing a pincher grasp can take some time. I know for Adeline she "raked" her fingers for a couple years before she was a full time pincher.
I had an empty shoe box and today I made a "pull box" to work on pinchers. I poked a hole from the bottom to the top with a knife and strung ribbons in the holes from the bottom to the top. I tied a knot on the bottom so the ribbon could not come all the way through.
The bottom of the box.
The top of the box - ready to be pulled.
Pulling the ribbons.
This is a pretty simple box to put together to help develop those needed skills.