After I installed GWX I can't keep lock for more than a few seconds.

[CjStaal]

Originally Posted by UKönig

Sometimes it's your weapons officer. If he is not properly qualified, he will lose his lock ons at range. When the ship gets closer, like average range for the torpedoes you have loaded, the lock will stay solid again.

What should I qualify him as? A torpedoeman?

[UKönig]

I believe so, yes.
Visibility is also a factor. If skies are clear and the seas are (reasonably) calm, you can lock on and stay locked much farther away.
Sometimes fog is so bad that even at spitting distance your WE won't keep proper track of the target.

[Hambone307]

I wouldn't worry too much about losing the lock if you are at 100% realism. It affects you more if you play like me and have auto-targeting enabled. Other than that, Ukonig is correct. Weather and sea conditions affect how far (or close) you must be to maintain the lock.