Some frogs get bored with a food item if Fed mostly that item, but it could also be something wrong with the worms. For example say that you've always been buying the same brand night crawlers that are kept in a particular soil and for some reason they're out of your brand so you purchase a different brand, but these worms are kept in a different soil. The new brands soil could make the worms taste differently and if its a bad taste it could turn the frog off eating night crawlers period due to the frog remembering the bad taste and making the connection that night crawler equals bad taste. This can happen.
Another example are Red Wigglers which release a noxious secretion when injured or when they feel threatened. Frogs will eat them until they get a taste of that secretion and then will not eat them anymore. This happens a lot with Red Wigglers.