Does lime juice make jalapeños 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.