Same thing is happening to mine. Not sinking nails much more than halfway (using 3'' nails), and a clicking noise which follows.
Sent in to hitachi for repair, received it back a few weeks later and there was no difference, same problem occurred.
They claimed that they "recharged air cylinder" and "adjusted nail depth". I believe they did those things, but unfortunately it did not solve my problem. I made the (terrible) judgment call not to get a warranty when I purchased this tool, so I am in a pickle not knowing how to have this tool repaired. At this point I would gladly pay a repair fee if I new the tool could be fixed.