Gravity. The shower head is probably 4 or 5 feet higher than the bath tap.
Diverter not working properly.
Employ a plumber.
Is more water comming out your tub spout? if so change spout
More "VOLUME" flowing from the tub spout then the shower head as the shower head has a flow restrictor
depending on your spout diverter if its a valve or is it on the spout it self, if the diverter is on the spout then your diverter is not letting full flow of water out of the spout and its putting enough pressure to allow the water out of the shower head, I had this problem before and if you change the spout it should fix the problem. If you have a 3 way valve which is on the wall then i would suggest replaceing the parts in that valve if possible other wise change out the divert valve. If your broke like me and want a cheap fix then put either a water flow resistor or a shutoff right before your shower head.
1- plugged shower head 2- defective diverter
if its just a shower and no tub spout then it most likely must be in your shower head and if its a hand held shower head (with a flexible hose attached) it most likely has a bad seal or some other way which allows air to get inside and drain the water out slowly. The same can be for a shower with a tub spout diverter and the shower head has a bad seal thus letting the water drain down the tub spout and it can be a real slow leak to cause the squeaking noise 20 minutes later, my guess of the squeaking noise would come from the air being pushed inside the shower head.
Your tub spout isn't fully diverting the water to the shower head and a very small amount is leaking through. The smaller the hole the water has to exit, the greater the pressure. Replace the tub spout, usually costs less than 20 dollars.
the tap assebly could be cross connected if older shower set
A power shower head mixes moving air with water, to increase the strength of the water coming from the shower head, while using less water. The advantages are lower water bills, using less water and protecting the environment. Some reviews of power shower heads indicate that the claims are untrue, and that the product does not work as is a waste of money.
Do you mean water leak or water pressure at the faucet? If you are not getting water thru the pipes you may have low pressure, or there is some blockage in the pipes preventing the water from flowing. Lots of times dirt gets in the small holes in the shower head and the sinks faucet (right where the water comes out). You will need to turn the water off in the basin or the shower. Next, take a pair of pliers and remove the shower head, or the basin's spout head. Look in the piece you removed. Is there dirt in it? Clean it or replace the part. Turn the water back on. Hopefully it was only debris blocking your water flow. The entire faucet will not have to be removed. Only unscrew the spout's aerator head., or the showere head.
There is rust in the steel pipes of your house, possibly the water heater. Have them looked at soon
When you pull the stem up, it diverts the water from the tub spout to the shower head. Somewhere between the valve connection and shower head arm, you have a leak. The easiest part to check would be the arm that comes out of the wall that you screw the shower head on to. It is normally screwed in to a "shower Ell" and that connection and or pipe is easy to crack if someone pulls on the shower head. Pull the escutcheon plate towards the shower head and try to see into the hole and look for water at the connection. It would only leak when you are using the shower. If it is another connection or the riser pipe itself, you'll have to access it through the wall, hopefully from a room on the opposite side of the wall. Good Luck