If you are referring to distant contacts that pop up on the map, those are radio contacts. They only show the contact's position as of the time of the report (which you can find out by clicking on the contact), and they get lighter as they age. When you get a contact like this, you find it not by going to that spot on the map, but by figuring out where it will be by the time you can intercept it. Use the map plotting tools!
It's a dynamic campaign, so ships and convoys all over the world are moving all the time. Just because a contact was revealed to be in one spot doesn't mean it will stay there and wait for you to sink it
If you're talking about nearby map contacts, then is the same - a contact is solid colour only when you have direct visual on it, and as soon as you lose sight of it, it starts "aging" and getting lighter. Get closer or make periscope observations to update it.