My dogs are still digging on our yard but they have a sand pit?

i went to our back yard there were new holes and i just went like an hour ago there was nothing. i was playing fetch with them but when i walked out i saw a hole why are they not using there sand pit bt still my backyard still kinda annoying me.. any help ty the only way they use the sand pit if i put treats on the sand pit then i leave they dont use the sand pit at all....