You are right, you should be getting more eggs from them each day.
A couple of thoughts:
First: Do they free range? Could they be laying their eggs someplace that you don't know about? I found an egg behind a hay bale in the barn one day.
And, I accidentally saw part of an egg sticking up from the pine shavings in the corner. I checked and one of my girls was laying her eggs in the corner and then burying them! If I hadn't have seen that egg sticking out, I would have cleaned out the coop this spring and found who knows how many eggs buried under the bedding! Every day I had to dig in the shavings to find her egg. Silly chickens.
Second: Cut back on the treats. Stick to layer feed, with free choice oyster shell and grit (if needed) and only give an occasional treat.