Tile Grout in Bathtub drain
I can't understand why CLR wouldn't work after all that time, unless it's no ceramic tile grout?
Is there even an improvement?
Do you have an idea as to how far down the blockage is? You might be able to guess relatively close by letting the slow drain empty and seeing how much water it takes before backing up. Locating the clog could help you if you have access to the tubs trap or a cleanout plug in the drain line. If there is no known plumbing access and the CLR isn't working, although it should, you may have to call in a pro if you or your husband isn't skilled in plumbing.
One more question - have you tried leaving it along for a while, to let the slow drain drain down as MUCH as possible beforre starting again with the CLR? IN other words, so you aren't pouring CLR simply into a drain full of water.