Originally Posted by Paul Eggleton

Could be all manner of things. You're not going to know until you take the rocker cover off.

Edited to ask

a) Did you replace all the rocker nuts with new ones and torque them up correctly?
b) Was the belt tensioned properly and has it slipped any teeth?
c) Did you put the spacer plate back in properly?
d) was the cam brand new and did you do the right running in procedure?