Hi Marc, Thank you for your question. Most likely, you are experiencing "shower-rise" where the water can’t get out of the spout as fast as it flows through the valve. This causes the water to back up and come out the shower head. There can be one of several reasons for this:
The valve could be upside down. The length from the valve to the tub spout is outside of the 8" to 18" rule
There is more than one 90-degree angle
Something other than copper or galvanized pipe was used going to the spout
The most common reason is that there is something restricting the water flow to the spout, e.g. solder or something lodged in the pipe. Something is impeding the flow of water to the spout and causing it to back up and go out the shower head. If this is the cause of shower-rise it is possible to remove the blockage by removing the tub spout; then, feeding a plumber’s snake or a speedometer cable up through the tub drop. The act of feeding it causes it to twirl and loosen the blockage. Be sure to flush the valve before reinstalling the tub spout. Be sure to check the tub spout itself for blockage.
If none of these help, you may need to call a plumber. After the shower has been installed and the wall has been closed, there is little that can be done to remedy this situation yourself.