Jabsco sea toilet.

I am sure what Daydreamer and Plum have said is correct

When you move the lever to the dry bowl position it is not lifting the outlet valve . Maybe you are not moving the lever fully to the right. maybe someting is broken or incorrectly assembled

See the Jabsco Tech video
How Does the Pump on a Jabsco Manual Toilet Work? | Xylem UK
To see how this should work

Also see the video which shows how to replace the "top valve gasket"
Jabsco Manual Toilet Top Valve Gasket | Xylem UK

Check that the finger #20 which should lift the valve is not broken or damaged in any way and that it will move freely when the external part # 23 is operated
( is #23 fitted to #20 correctly?)

View attachment 121393
This sounds like the issue is part 20 is not seated correctly, I’ll have a look next time I’m at the boat, many thanks.
 
Top