When I flush my downstairs toilet it is working in reverse. When I pull the handle the bowl fills with water and then slowly drains back into the reservoir but never actually flushes. What do I do to fix this issue and what could be causing it? The other toilets are working fine.

Sounds like you have a blockage in your waste pipe.

You can either unscrew the toilet and pull it away from the 4 inch waste, which you will be able to examine the waste pipe to see if it is blocked that end.

If no blockage is visible there maybe one further down the line, depending whether the pipe is a straight run to the down pipe, you may be able to dislodge this with a set of rods and plunger.

Could be a clog or it could also be deris in the small hole at the base of the toilet. This hole flushes water to start the siphon effect, check it for debris, gloved hand works. I would also plung the toilet and if need be use a toilet auger.