I have a Bosch WFF 1401. I removed the drawer as I

There can be 2 reasons. The centre section dispenses the fabric softener in the last rinse. It works on a syphon effect.
If it is not assembled correctly the water flows over the side to avoid a flood and leaves the middle section full at the end. Make sure the inner pipe is inside the outer pipe and that both are completely clear of softener residue. As you have just reassembled it this is the most likely cause.
The second reason is the water inlet can become blocked therefore not creating sufficient flow to create a syphon effect. It gets blocked with black gunk and needs to be cleaned with pipe cleaners or a long cable tie. There are 4 rectangular holes above the centre section, near the front. They can be seen easily if you take the drawer out.

Hope this helps

Nick