I keep getting detected by escorts??

[holohw]

i cant seem to make an approach at periscope depth with the periscope down (except for collecting target data) without the escorts picking me up and the target ship starting to chamge course and run off leaving my attempt at setting up a solution in shambles.

Any tips? I am using WernerSobe basic method, the second video in sub skippers bag of tricks, still getting to grips with it