What is a stacking issue in the shower.water comes out of both the showerhead and tub spout.I was told it could be the deverter,its not.I was also told it was a stacking issue because my house is plumed from the ceiling down and I need to loop it so it comes from the bottom up.I don't understand how this makes a difference?They said it's a stacking issue.

How long should or short should the pipe for the tub spout be coming out of the wall

water comes out of both the showerhead and tub spout Is your diverter on the spout or in the shower valve?
What is the PSI in your house?

How long should or short should the pipe for the tub spout be coming out of the wall
The length of the spout stub out depends on the type of spout that you have installed. Spouts come in two flavors. A slip on and a screw on spout .
Hint! If your spout has a opening in then bottom ,(see image) then you have a slip on spout and the stubout will only be 2 or 3 inches long . What do you have? Back to you, Tom

I've read your response concerning deverters and water pressure.This should not be the issue sense my second bathroom works fine with the pressure I have and I have taken the deveter (spout) off and it does the same.As in my original question I was told it was a stacking issue? Because my bathroom is plumed from the ceiling.

It's a screw on type

What difference does the length make

Plumbed from the ceiling is not the issue.

Question: How high is the shower head above the h/c valve... and... What's the pressure in your house ?

Please, respond... Back to you... Milo

My pressure is the same as it has been for the last thirty years.Both bathrooms worked fine before this new fitting and my second bathroom still workes fine with my current pressure.Could you explain this?Thank you for your time!