Genetics, which at this point you can't do any thing about. And letting them get fully ripe helps. Other than that I don't know. I've heard that stressing the plants can make them hotter, but I've never heard any hard facts about it.

Excess nitrogen can "cool them"

Sulfur in the form of epsom salts is often recommended, especially dissolved in water and sprayed on the foliage every couple of weeks. Also, keeping the plants dryer or warming the plants up is supposed to help.

Contrary to the general rule I've heard, I've found serranos to be somewhat milder and sweeter when fully ripe.

All of my peppers turned out mild this year too. I grew habs that tasted like bells and almost all of my others turned out sweet tasting. Don't know waht it could be.

Add epson salt to the ground, stress them out, try not watering foliage just the ground.let mother nature water foliage. Jalapeno and Serranos fully ripe take on a softer flesh and a sweater taste but still should have a burn. Growing for 35 years
bowhunter

Michael Bailes the Austalian author of "The Fragrant Chilli" claims that adding a little detergent to the water will make peppers hotter.

Keep in mind that chiles cross-pollinate freely. This is why you may often have chiles from the same plant with varying degrees of hotness.

I had a buddy of mine give me a bag of habaneros...'cuz he said that they were WAY too hot. Got them home and asked one of our sons if he'd like to try a really hot pepper...cut him off a piece and he chewed on it. He said "Not hot at all...nice flavor but definately not hot". So I cut a piece from another pepper...he tried it...same reaction. I tried a piece, no hottness at all.
So the next day, I chopped up a bunch of 'em to add to my eggs...HOLY MOLEY! Good God...get me some milk, now!!

lol! I thought that kind of stuff only happened to me.

I am growing a Ghost Pepper plant, supposed to be the hottest pepper you can grow. We picked one a little prematurely, it was still green. But it wasn't even remotely hot. I don't get it. I figured the hottest pepper you can grow should at least have a little sting to it.

HELP!