Check your valve clearances! It's a pretty easy process, just a few tools, and takes around 30-60 minutes. You should really get in the habit of checking the clearances every 10-15 hours, or any time the bike gets abnormally hard to start.
The CRF250's are known to have the intake valve clearances tighten over time, and cause hard starting. If they are tighter that spec, then you will need to put in smaller shims to bring them back into spec. And rule of thumb, if you reshim two times, then you might want to consider new valves/springs.