Have you only had them introduced indoors? I remember seeing a segment on a dog training show that introducing new dogs in a "neutral" territory is better sometimes. B/c the dog that was already living there won't feel s/he needs to protect "their" home. Of course, use positive reinforcements to show that having the new puppy = treats/toys
I'm a fan of Victoria Stilwell.
Hope it helps. Good luck!
https://positively.com/dog-behavior/...existing-dogs/