I have a trip lever drain in my bathtub. Thought it was the problem, but it's completely out and the tub still won't drain. Any ideas?
I would start with a basic hand operated drain snake. In a tub drain you likely have a hair clog, the snake will grab onto this clog and pull it right out. A very basic snake should be all you need.
Great answers so far.
To unclog the drain in my bathtub, I've occasionally used a toilet plunger by placing it directly over the drain then filling up the tub until there is enough water to create suction.
A few good "shots" / "pumps" should unclog anything that is stuck in there and definitely worth trying before purchasing any liquid drain products.
If it's a hair clog, I've had great luck with a "Zip-It" plastic snake. It's only good for close clogs (20 inches long) and has barbs that catch/pull the gunk out when removed. Should be a few dollars at most hardware stores.
You can buy proprietary drain unblocker products. They are often a sodium hydroxide based product (so take appropriate care) that is combined in a dense gel that is designed to sink through the standing water to be able to work to break up the blockage.