Sometimes you have to get them a taste for the worms by sneaking in while the frog is awake and either slowly move the worm piece twards him with tongs or drop the chunk on the substrate directly in front of him about an inch away from him. Most will strike without thinking. Also Red Wigglers are NOT the same as Night Crawlers and secrete a distasteful slime that both tastes and smells bad which is probably why your frog wouldn't eat them.

About cutting the worms, just cut a half inch to 1 inch piece off of one of the Night Crawlers starting at the tale. Only cut one piece off each worm at a time. They can regenerate and therefore are not harmed much by doing this and will survive so the whole worm will not die and be wasted.