It's obvious to say this, but working with someone over and over again, builds that higher level of familiarity. You get to know what their strengths are in both posing, attention to details, working practices, etc. With this familiarity, when working at either comic con events or on location, there is an established expectation of what can be achieved already before the first photo is taken. Also on the skills which aren't as strong, both parties can either find time to work around that or work on it without pressurising each other.
Level of Quality
There is something which I'm experiencing when I am asking cosplayers for their time for a photoshoot but don't be seem to but other photographers are. I suspect why I am not working with all cosplayers is because, cosplayers want a certain level of quality for their portfolio.
I keep stating this, I'm not saying I am at the top of my game, I know plenty of others who are better at me. I know my work is a bit up and down, in quality and I am working on that. But some cosplayers and photographers want to make sure, the output of photos is of a certain level and which there will be some disappointed people who will get the gentle shake of the head.
What Can I do
Well, like anything, if you want to work with certain people, just got to keep trying and asking. But also, never stop improving oneself, because if someone doesn't value your work now, others will and if you are happy to work with others, then you are helping each other learn and do more.
I wrote a post about "why no one takes my photos" going into that subject further.
Let me know what you think about what I just wrote, advice you'd give others. Also if there is a subject you like me to write about, do let me know.