Gracie has this problem upon ocassion. Try some metamucil (unflavored powder) sprinkled on thier food, for 6.5 lb dog I can use 3 tsps daily.
She did need a little extra help a couple of weekends ago so my vet gave me Laxotene. It is actually used to prevent and aid in the digestion of hairball in cats. Sure enough one tsp of that and she was able to go
It comes in a tube that looks like nutrical.
Good Luck