Colorado49

My son has a Aerox 50 2005 which has been a pain since we bought it in July 2012. It was OK for 2 months but since has been slowing up and sometimes not starting. It has standard head, but has Leo Vince exhaust. When it first played up, i bought new plug, coil, carb none fixed it , ended up buying new battery. It has 2 main problems, it slows going up hills and once speed lost does not seem to want to pick speed up again once on flat. The other is he can ride for hour stop somewhere for hour, when he tries to start it refuses to fire. At the moment it wont start again. Spark look fine, there is fuel getting to carb. If you remove plug and dry it does fire but not start and then seconds later it doesnt fire at all, as though its flooded. Removed reed valves and they seem OK. So hence I am here desperate for help, I have spark, fuel and there is compression so why wont it start ?