Why does this happen? It barely flushes, if at all! It is an add on, in the master bedroom. Was done before we moved in. I don't know if the folks who did the work were professionals? Yikes...

Also, water comes up into tub when washing machine drains. I live in FL with a septic :) Help?

Many possible reasons

For toilet ( could be all the other plumbing reasons below also,
But could also be.
1. improper drop in pipe
2. improper pipe size
3. blockage in the pipe for toleit
4. no vent pipe

For all the other issues,
1. blockage in drain pipe
2. septic tank is full and needs to be pumped
3. field lines are not working properly and septic system needs to be updated.

Megtre, has this always been a problem? Are you on septic or city sewer? If septic has it been pumped lately? If city and your looking for a do if yourself correction of this problem, I would start by running a snake in your main drain look for clean-outs to snake from. Hang tight and an expert will see this soon. Good luck