I make guacamole with a whole jalapeño and the last time it was hard to mash so I poured lots of lime juice in this time. It was hardly spicy at all. Did the extra lime juice make it less spicy?
Lime juice isn't going to make it less spicy, if anything I've found it accentuates the spiciness a bit although I'm not sure of the mechanism. It could be that the acidity frees up more capsaicin compounds (what makes hot food hot), or wakes up your taste buds more.
Most likely you have simply added a weak jalapeno. Peppers of the same variety can vary in heat strength quite a bit depending on the where and how it was grown, although you'll get weak and strong peppers from the same plant. You can make up for the lack of spice by adding more jalapeno or a bit of hot chili powder.
I see all the answers leaning toward the negative but wonder whether the responders actually tested their theories. I recently had a similar experience with habaneros and lime juice, that is I added my usual number of habaneros plus seeds to salsa, but used two noticeably juicier limes and my salsa had almost no heat and the flavor of lime was much more prominent. I know this is only one example but it was noticeable enough to make me Google the possibility of lime juice cutting heat. Anyone else with similar experiences?
A little more hunting turned up this:
If you follow the hot peppers with an acidic food or drink you can neutralize some of the activity of the alkaline capsaicinoid. Good choices include cold lemonade, a lemon or lime, orange juice, anything tomato-based, or drinking milk (which is acidic).
Information from How To Make Hot Peppers Stop Burning
When cooking chili, and your chili heat is too hot, it is suggested to add fresh lime juice or a little brown sugar to drop the heat down.