yup yup, it has happened to me countless of times.... Here a few things to notice while shifting, especially on the 250, but on all bikes in general:
1.) Make sure to disengage the clutch completely before you shift.... This doesn't mean you have to be slow about your change, just pull on the lever a bit more.
2.) pre-load the shifter before the change - not a ton, just kind of rest the weight of your foot on it right before the change.
3.) When you do change, make sure it's a solid movement on your part. No limp-ankled, half-kind-of-sorta-blah shift. A solid, succinct kick-down with the foot. But don't be all hulk on it either and kick with your might. Accurate and succinct has the magic, they happy middle ground that is.
Also, try some clutchless shifting, it gives you a whole new appreciation for feeling what the transmission is doing.