Oultlook Web Access

I have a question about owa. Currently my users are out ot office and are checkig their mail through owa. everything was going fine for the first few days, however on the fourth day no one was able to log on. At the authentication screen they would type in their password but it would not accept it. I found that the only way to let them log on was to give them domain admin rights. Now I dont want to give domain admin to regular users. I was in the same situation a few weeks ago and I am still unable to find the cause of this problem. What could be the cause of this