Sounds like you did not get an even coat on. Of course making sure the valve cover is perfectly clean and no spots are on it's surface are important. If you are using reflective chrome you need to apply a powder primer. When applying powder make sure and go over the valve cover several complete times, even 3 times. A good coat.
When you apply the primer (1 good coat) only bake it untill it starts to flow then take it away from the heat source. Do not fully bake it. Apply the reflective chrome next after about a 5 minute period and lay on several good coats. Now bake it and when it starts to flow, run for 12 minutes. Keep in mind you have a primer that has run 5 minutes so adding 12 minutes on the chrome keeps it from over cooking to where it becomes brittle.
If you are using a infra-red lamp bake from the back side of the valve cover to avoid heat spots. i.e.-- if you bake with a lamp unless it is big enough to cover the complete surface of the valve cover and flow it in one complete step, the results from the smaller lamps is that it only bakes 1/2 of it then you move the lamp to bake the rest and it leaves a streak of where you stopped & started.
Just always remember- Reflective chrome needs a primer base.