Mike, I would look into what soil you're using. Do you have proper drainage? That's really important. Your plants should be bigger for almost 2 months outdoors. How much sun do they get? They need at least 5 hours of direct sunlight per day.
Go into some more detail about your grow parameters so we can help you help yourself.
I've seen plants like yours this year....like my own. I didn't get proper drainage, and the plants just stayed at a particular height and wouldn't grow any bigger.
Transplant the plants into a soil that has a higher percentage of one or all of these soil parts: peat moss, perlite, or vermiculite.
Also, if you're using Miracle Grow fertilizer, I'd stop that, at least for now. You didn't specifify if it was Miracle Grow fertilizer or soil.
One more thing, make sure you have drainage holes at the bottom of the containers you're using.