What could cause a dell laptop on a dell d dock station not to lock?

i have more than one type of laptop but all the lap tops use the same type of docking station. Its the dell d dock. The computers are set to lock after 15 mins but some wont lock on the docking station. Is there something in the docking station that could be stoping the computers from going into idle state?What could cause a dell laptop on a dell d dock station not to lock?The docking station is like having extended devices connected to your computer. If one of them is not setup correctly or is having trouble, it will cause strange things to happen.