The standard response to anyone playing v1.4, seems to be to get the u-boat missions/v1.5 upgrade. Since the majority of mods are written for that version it's well worth doing.
Can your sonar operator still pick contacts up? I've been unable to hear anything when using the sonar screen manually, but my soundman has been able to report on them. I've heard someone say that the target won't appear on the hydrophones if it's moving at less than half speed.
I've had a look in the downloads section to see if there's any kind of game fix or patch that addresses this, but I can't find one. Anyway, there's a great deal of expertise hanging around these forums, I'm sure someone with the knowledge will be able to shed more light on this than me
Happy hunting!