Hi Sammy-rai
1. is your engine room fully manned or engine room crew fatigued
2. Have you rigged for silent running by pressing "Z" Key if so you will be doing only 2 knots at periscope depth like in screen shot below with the red border around your propeller icon Sir
please could post a screenie of your activated mods so I or any other time served Good Egg will be able to give you a heads up so to speak and point out whats causing that issue