shims are missing??

[WB450r]

so this past weekn i replaced a head gasket on my 450..i put everything back to normal tightened all the bolts to spec etc..cranked it up at the track let it warm up for 5 to 10 min, rode it around in the pit area for a couple min then cut it off to check for leaks..no leaks..5 min later i got all geared up and went out on the track..quad rode like **** so then i pulled the top of the head off and the 2 shims that go under the rocker arm by the cam are missing..anybody ever had this problem and what could've caused this??

thanks