Noticed a lot of players having issues getting their tames impregnated. Found a tip online that we tried and it worked nearly flawlessly.

Trap the female rhynio. Using a pteradon works great at pulling aggro.

Get the rhynio down to under 100 hp, NOT 10% of its health! Feed the pheromone to tame, dismount tame and release the female.

She'll aggro to it for a moment then start to fly off but should come right back. (If she continues flying off, use ptera to regain aggro and bring it back).Give it some time, and she should back up and do the animation for impregnating, then instantly fly off. If it was successful it should tell you a timer for incubation as well as the next craving.

Tried it with three bronto (m/f), and it worked with just a single pheromone. Ended up using that female about 15 more times. It seems to have some issue with impregnating rexes, though, as it takes longer for them.