why German doesn't attack?

[hempy]

some times German's U-boot try to attack my supply in ocean,, but some times they come to do nothing, but some times attack, why?

[mikwarleo]

There are a lot of reasons why they might not attack. I assume you haven't played many games as Germans yourself yet.

Basically any unit in the game can only conduct one attack per round. So they might have already attacked. Or they might be out of torpedoes. In WaW Uboats have a special ability which allows them to occupy the same sea area as enemy units without the condition that they must engage in combat.

If you read the manual some more and play a few more games you'll get a better picture of how this works...