I have a bad tooth ache and was prescribed Amoxicillin 500 mg but ita not working?

Take something for the swelling like Aleve or Advil, call your Dentist soon as you can, he may give you something stronger.

I personally have a resistance to amoxicillin/"Step 1" antibiotics, and have used Z-Pak (Azithromyocin) several times for tooth infections with success.
The antibiotic Augmentin has worked well for this, too.
Both are considered 'stronger' than Amoxicillin.
I would definitely call your dentist ASAP to let him/her know the amoxicillin is not working properly & then go from there.
Swishing with warm salt water can help somewhat, by the way! You can do this every few hours.
I was also always given a mouth rinse when I had a tooth infection etc. called Peridex--may want to ask your dentist about adding that to the new antibiotic as well.
Hope you feel better soon!!!