Mayonnaise gets too thin after adding solids
I'm going to step out on a limb based on experiences I've had. I suspect that adding garlic and or chopped clams after you have emulsified your other ingredients is simply adding moisture and making your mayo/aioli thinner. (Especially since you said that it doesn't break.)

I would suggest putting your garlic or clams in at the beginning with your other ingredients before you emulsify with the oil. If you still find it to be too thin you can whisk in another whisked egg yolk or a little more oil to get your desired consistency.

Re the comment about acid, I don't use any. Also there would likely be vinegar in the Dijon mustard, so you are not sans acid.