I have had this problem twice with my A50. The first time I took all sorts of things apart to try to solve it. I cleaned every part of the carb, fuel lines, and air intake. Nothing helped so I replaced the spark plug and checked the electronics. After all of that it still didn't get better. I drained the gas and added fresh 93 octane as a last ditch effort. Boom everything was solved and the scooter road exactly like normal again.
The second time my scooter wouldn't rev up much past idle I went straight to the store and hot a bottle of ISO-HEET Fuel-Line Antifreeze, Water Remover, & Injector Cleaner. Dripped an ounce or two into my gas tank and shook the scooter a bunch until I felt like it was mixed well. Fired it up and let it idle a while until I felt it was recieving the treated gas. Sure enough, the scooter ran like normal once again.
If you're ever having trouble starting a scooter or getting it to run like normal after a fill up, your easiest and cheapest option is to replace or treat the gas. When that doesn't work then begin to dig into the mechanics and components.