why do you need to summon one? you could [and should] just astral project and talk to them yourself. if you need one for a spell, just call for the dragon to help you instead of a deity. if you need them to charge an item, leave it on your altar overnight in case they come by to enchant and object.
summoning usually pisses them off, so be sure you know which dragon you need, and have a good reason. if you want to summon one to see it, or because you want your ex back, they'll most likely attack you.