We had a leak and some pipes had to be changed that lead to the shower head. We also had a new tub faucet installed. Pressure was low to both the shower and tub so we had the plumber return for an adjustment. It was OK for a while, but still not very much pressure. Several months passed and now and the water barely flows. We are able to do some repairs ourselves, but where do we start?

If you have galvanized pipes it's possible some "stuff" has loosened and is clogging the outlets. You may have to remove the tub faucet cartridge and run the water, flushing it. Shut off the water first. You can remove the shower head and try running that to see if anything comes out. Since it's connected to the tub faucet this might clear that up too. For that you don't have to shut off the water.

Shunned went right to the cause of your problem. Crud that's built up on the pipe walls was dislodged and now it's clogging the inlet ports of the faucet and the supplies. They will have to be cleaned and flushed out.
If you need assistance in opening up the tubs faucet and flushing out the supplies then we will need the brand name of the faucet. How old is it? What kind of pipes do you have? Copper? Galvanized?
Let us know. Regards, Tom