Sounds like a stripped sear. My advice is get a new Trigger Assembly or see if you can't get a new sear kit. I tried googling for about a quick 2 minutes and didn't find anything, but I'm sure if you check Trinityairsoft or AirsoftGI or Redwolf you shouldn't have a problem finding it, or at very least, finding a replacement trigger assembly.
Also, I saw you posted the same question on AirsoftForum. Listen to the advice that guy gave you, he's dead on.