bath tub stopper not stopping water

My tenant's bath tub was completely missing the flip up bath tub stopper cover and what was behind it. Not sure why she would throw it out. I purchased a brass stopper which is shown. When I lower it down behind the cover, it does not stop the water at all. It drops down to what seems to be the bottom. Is it possible it is not the right size? or, is it possible there is a piece still in it blocking the new one? It seems to be going down all the way. I can't see down into the area and there is no access anywhere.
Any suggestions? is it maybe not a tall enough drain stopper? I tried to use a hanger to try "lifting" anything that may be left there but didn't feel anything.
Looking for ideas.

 

I'm guessing the stopper is too small in diameter. When you drop it down the hole, does it just fit into the down pipe or is it smaller and loose? If it's the right size it will just fit.

You should be able to estimate the length by measuring inside the tub and adding a couple of inches since the drain tee is normally a couple inches below the tub.

It might be easier to replace the drain with the type that includes the stopper, such as this one:



But it can be a bit tricky to remove old drains that have been in place for a long time. Technically you don't need access from the bottom to do it, but if anything goes wrong, access will be needed.

 

You may be correct that it is too small a diameter. It did seem loose as it is lowered into the hole. It said it fits price pfiester. I will double check available sizes and also if it goes down far enough. it seemed close to where it needed to be. After seeing how hard it can be to undo the drain, I wanted to try this first. thanks for getting back to me. Back to the big box store.

 

I found all the missing parts except the actual round part that stops the water from draining. I think that part broke off behind the tub. There is no access. I tried fishing it out with a hanger but no luck. I can't see down there and wish I had a colonoscopy camera.
Will try again to get it but will try to replace the drain with a drain/stopper which I bought. I also got the drain remover tool.

 

The drain removal tool might not work. You can cut the drain flange fairly easily using a hack saw blade.
Hold the blade with a glove or rag and cut the flange and inside body.
Avoid cutting into the internal shoe threads, the flange will pry out without cutting all the way through it.