cranks but no start

[newone22]

I am brand new to this and i hope to do this right.got grandkids taotao 125 new cheetahs about 3 years ago. I have very big back yard and they have good place to ride they have not been put through the old ringer. the one that has and is giving me trouble is the boys 125 it keeps getting harder and harder to start and now won't start.i am some good when it needs work. it has a new battery and a new plug and wire, solenoid, regulator, cdi box and have checked the valve clearance and it is good.i did this twice and the machine started and ran good until it is turned off and then it will not start what so ever.it did this twice. this is where i am at now.i hope i can get some info on this before i go crazy.thanks very much randy