My shower drain has been slow for quite some time (as in years). I'm generally able to get it moving with a couple quick plunges (specifically pushing the plunger down and then pulling it up hard). No one else in my house seems to be able to master this technique and as a result they end up standing in a couple inches of water while they shower. The drain has a built in grate that is about an inch below the floor of the shower, and is not removable. There is a removable grate at floor level, that we keep in place other than when plunging the drain (it catches hair and such). The openings in the fixed grate are too small to run a snake down the drain. I did use some thin plastic dohicky that had a pieces of Velcro on it - didn't fish anything significant out that way. I have put Drano type stuff down it many times, with no notable change to the drain. Additionally, my boyfriend tried sticking the shopvac to the drain - it made no difference whatsoever.
No other drains have any issues in the house. (2 other bathrooms, various sinks, laundry, etc.)
Any suggestions or ideas about what might be the source of the problem and how to go about solving it?
Is it possible there is a bend or curve in the drain pipe that could catch an air bubble and keep the water from draining - which is fixed by my super-secret super-power plunging? Something else?
As far as I can determine the drains are not accessible without tearing out walls in the finished basement, or tearing apart the tiled shower floor...