There is a simple way to diagnose weather the toilet needs repair or the main sewer is slow in a weak flush situation like this. Fill a 5 gal. bucket with water and pour it into the toilet bowl rather quickly. If you witness a good flush the toilet is the problem. If it is still weak you have a drain problem. Sometimes toilets get calcified up around the rim holes and the main siphon jet. This doesn't allow the water to exit the tank and enter the bowl fast enough to siphon.