[A bones]

Full of fluid and not leaking does not guarantee the workings of either the master, or the slave.

Typically a failing of either cylinder will show up as you describe. A gradual loss of service resulting in harder shifts. Particularly trying to engage first gear at a standstill.

To verify the action it helps to have a buddy work the pedal while you measure the action at the slave. Or even better if you remove enough flooring to view the slave.

First thing though, I would try bleeding the slave to remove the possibility of air having gotten into the picture.

Good Luck.