Yeah i have the same problem with mayonnaise for potato salad... when i add other things to it, like spring onions, a bit of feta, or dill, it goes really really thin. Last time, it actually complete separated... wasn't too appetising having potatoes sitting in a pile of oil!
It's heartbreaking when you've waited overnight while potatoes cool and what was a perfect looking mayonnaise is completely destroyed :/ Have been reading a bit about it, and I think "cold" might be a problem. Also different mustards make a difference to the emulsion... one site reckons whole grain is the best to use as the emulsifying agent is most prevelant in the husk of the mustard seeds (i.e. in the "whole grain"). Dijon was a close second.
But for me i think i might have my fridge too cold... I usually keep the mayonnaise in fridge overnight and ice crystals might be forming. Or I keep the spring onion and other ingredients in the fridge, so they too might form ice crystals.
As a side note: I wouldn't use olive oil for mayonnaise, it's far too bitter but I think I might be especially sensitive to bitter tastes (e.g. i think the world is weird for liking beer ha!)