Frustrated. How to order a hamburger in Mexico without mayo.

(EDIT 2: Thanks for the tips, everyone! I went to the puesto around the corner, watched while she made the burger, and when she was about to put the mayo on, that's when I said "sin mayonesa, por favor" and it worked. Woohoo!)

Yes, I know this is a stupid question. My wife and I have lived in Mexico City now for about 18 months. She's a dual citizen and I'm on my way to being a permanent resident (have my temp residency at the moment). Over the past 2-3 years she has learned Spanish to an advanced/near expert level (not fluent yet, but she's working on it!) and I'm still at a beginner level.

Anyway, I love burgers. Yum yum. I hate mayo. Ew. Almost every burger in Mexico comes with mayo. Mexicans love mayo. Which is fine, I just don't. Five different times now in the past year+ at five different restaurants/puestos my wife has tried to help me order a burger without mayo and we always get questioning/blank looks and the burger always has mayo on it. You'd think "Sin mayonesa" would work, but it obviously doesn't. Again, blank and uncomfortable stares.

Is there something we're missing? Are we using the wrong words/sentence construct? God, I hate asking this question. It makes me feel like an idiot.

(EDIT: Thanks for all the quick and great responses. My wife's accent is very good (she keeps being told this) and she's half Mexican so she passes (I'm super gringo) so we didn't feel that was the problem and we always say "por favor". One commenter said it's just such an unusual request that it's like asking for a pizza without cheese, which I very much appreciate the analogy. We'll just be more insistent in the future. Thanks, everyone!)