That doesn't sound good at all....and please don't take this wrong - but Please do NOT use Hartz Products - they're known to be toxic and have killed many dogs and Cats.....Search the web and read about Hartz....I wish they were permanently banned from being sold in stores - too many people really don't know how dangerous this stuff can be
I hope your vet can tell you why this is happening because they shouldn't be losing teeth at that age and GOOD LUCK !!!!