Let me guess, you have 100K+ miles on your engine? Same thing happened to mine and to others on this forum. Engine vibration works them loose at around 100k miles. I first tried just tightening my bolts, then eventually replaced my valve cover gasket and grommets with a Fel-Pro kit that contained both gasket and grommets. I can't say I was wild about the quality of the Fel-Pro kit. I still have a tiny bit of leakage and the grommets started peeling around their edges pretty quickly. Maybe Beck/Arnley would be better. I went out and bought a new torque wrench for this job as my medium and large ones would not have been sensitive enough, so either use a light touch or use a torque wrench on the small end of the spectrum (inch-pounds, not foot-pounds).
Anway, as I12 said above, try just tightening the bolts and wiping everything down before you buy a new kit. And do be careful because you're dealing with aluminum, not steel.