Did the new valves slide easily in the guides? Could be a sticking valve which will "wear" in after a little running. I have had the same thing happen when fitting new exhaust valve in a Norton 850. It wore in and ran fine after a few days . A couple of thou can make all the difference with close fitting parts , or perhaps a slight bend in the stem of the valve that cannot easily be detected without rolling it on a flat surface , Turn the engine over slowly without the valve cover and watch for the sticking valve to snap shut . On the other hand , there might be excess clearance in one or two valves you didn't notice before. Good luck !
David Potter
