weird pcr product ask for help!

I did three batches of pcr last Monday and Tuesday. Got bands every time. Weird things happened when I tried a large batch of samples last Thursday. Target bands did not show in gel. But you could see that pcr products showed near gel wells. I tried new tubes of buffers,dntps,primers and taq for three times and it didn't work. I even tried the exactly same templates which got bands before. The result were still same. No bands at all. What could be the reason? Thanks
I'm new here. I don't know how to post picture here? I think the picutres of gel may help to know the problems better.

It sounds like this could be a problem with your gel or how the gel was run. Did your standard run the length of the gel or did it stay near the well as well?