Can I just stop you here... if you're seeing results now and change things just so you can feel sore, you run the very real risk of not seeing much actual progress. But hey, you'll feel like you did something.
If heavy low rep didn't "work", and light high rep didn't "work", have you tried heavy for high reps? Or high intensity with plain old ball-busting bodybuilding techniques like rest-pause, drop sets, trisets for the same muscle group, burns (going past failure with short ROM movements), etc.
Without knowing exactly how you're training everything (volume, frequency, etc.) I'd probably chalk it up to your individual muscle fiber makeup. It's not exactly a bad thing to see results without DOMS.