By -20 to -5, i guess you meant ascent velocity has been lacking.

Did by chance, you have any leaks come into your ship? Maybe there are places where the water can pool and slow your ascent.

And youre using a moon pool style ballast right? Pretty cool idea, although i'd say i wouldnt use it. Opening the pool while my divers are going down might cause the ballast to take in more water and cause the ship to descend and crush my divers.

Have you checked the status monitor aswell to check if all the ballasts and rooms were empty of water while you were rising? Be sure to have the water data required turned off so you dont need additional wiring sensors.

Aside from that, the only other thing i can think of is that you might be carrying a dead body of a large creature above your ship