Check your valve clearances at TDC and make sure they’re all in spec. If they are tight then remove the cams and reshim the buckets, it’s not to hard to do you only need a shim kit, some math smarts and a magnet to get the bucket out. 3 Reasons why your valves are getting tight, #1 the valve could stretch/be stretching. #2 your valve seat could be getting squished. #3 the valve lip could start sheering off. Now that last one is very unlikely to happen and if it did happen then theres a large possibility that your bike could start burning oil and smoking. Best of luck!!