Welcome to the site !!
First , you posted in the "Jetting" Forum , this should be in the "General" forum , OR in the "CRF250R" forum , but i will try to help you with this if i can....
Questions:
Is the engine at Top Dead Center ?? , and is the cam being inserted with the cam lobes facing UP ?? (the lobes should be facing 2-O Clock position , facing rearwards with the sprocket "notches" staying level with the head) *like this* _ _
Are the shims properly seated in the valve stem and the bucket seated down all the way ?? , and did you confirm the shims did not get mixed up or left stuck in a bucket and some buckets have 2 shims instead of 1 ?? , this would hold the cam up and not allow it to seat properly
When installing the chain , did you make sure the Chain Guide is seated properly and not putting tension on the chain ??
When you install the chain on the cam sprocket you should be able to tilt the cam into position and it should go right into place if everything above is done correctly
If any of these items are missed that could be the issue , hopefully one of these things is your issue ...... Check and reply back ......