You should be able to repair a normal dagger, make sure you have a repair kit and the dagger that needs repair is in your right hand. If that still does not work there is a problem and you may need to try repairing some of the looted weapons which have a lesser value than the standard weapons currently have. For example a dagger currently takes me about 55 seconds to repair but a dagger of stone is only 20 seconds. So one of the weapons worth 300 or less tria should be possible for you to repair. A dagger of stone is worth about 88 tria currently. It is all about the weapon value.
