There's several varieties of heatless Jalapenos.False Alarm and Senorita are just two. Any kind of stress will heat up the ones that are already hot.Withholding water is one way.Some commercial farms will actually flood the fields just before harvest to stress the plants too.
Try picking a few in the heat of the day and see if they are hotter. I understand the capsaicin content increases during the hot sunny part of the day, probably as some sort of protectant. Would be nice to hear from you about this. (Unless of course you really do have the newer sweet ones.)