There are two easy possibilities here.

I am assuming that you don't have this sub in current view by a naval bomber, which means it is an intel image. Depending on how old the intel is that sub could have moved a long way since then.


You can determine this by selecting the enemy sub. It will show a shadow spy figure in the corner of the unit icon, and a "intelligence from" line in the description


There is no guarantee that the owner of the unit has simply left the unit there for you to kill. The unit could have moved since the intel was gathered. In this case your sub is trying to attack, but there is nothing there to hit. Since subs can only be seen by naval bombers the intel image will persist.



Another option that doesn't sound like your issue, is that the sub could be still at the naval base where it was built. In this case the sub is hard to get to, because it is partly on land, not actually in the ocean.



These are hard to get your subs close enough to kill, as the water gets quite shallow so close to shore. Destroyers or Cruisers with a bombardment range are a better choice for these. If it is a surface ship and not a sub in the drydock you can sometimes get you sub to drive close enough by holding ctrl and giving it a move command to a point as close to the enemy unit as you can, and you might get it into combat range.