Ok before I answer, you should ALWAYS, before you even fire a shot, switch from AP to HE rounds! (AP rounds are useless, the only way they could harm the enemy would be if he overran your position and tripped over them!)
Sorry for the rant, but you are new to artillery and it is important you develop that habit.
As for not seeing any hits, there are two possible reasons.
1) Most likely the problem is elevation. This can be caused by two things:
(a) Elevation difference between you and the target. In other words the target is either higher or lower than you and you must adjust your elevation accordingly.
(b) The target has been sighted above ground level. For example, if someone targeted a church tower, and 'shot' the top of the church tower with their binoculars, then your elevation will be set to hit the top of the church tower. This is very common, if there is a building in the middle of your target area, try lowering your elevation until you see your shots.
2 You may, as has been suggested, be hitting something in the way. This often happens if you are shooting at a target in a city. This can be corrected with a slight adjustment left or right. Bear in mind however, that if your target is in a city then your elevation is probably wrong too as your target is probably a building, targeted at some point above ground level.