I got a few tips here, I made lots of mayo in school so far, aioli is one of my favorite salad dressings, though I use it more for sandwiches than anything else. Here's a few tips...
-The most important, froth the yolk before beginning your emulsion. If you just add oil to the yolk without frothing it good it won't emulsiy properly. I was having trouble one day and that is all it was, sounds like it may be what you're not doing.
-One egg yolk should emulsify 200 ml of oil, a few Swiss chefs told me 250 ml, so anywhere between there.
-A teaspoon of ground mustard per 2 egg yolks I believe helps emulsification.
-Add a little of the white wine vinegar and lemon juice in the recipe to the yolk/mustard froth, and as you are emulsifying and the mayo is getting a little too thick, add a little bit of the vineger and lemon juice to bring it back down to the consistency you want.
Hope those tips help, emulsifying can be a ***** but after you've done it a few times it's pretty simple. It's also difficult to emulsify small amounts, if you're only using 1 or 2 yolks try making a bigger batch (if you think you can use it all).