My whole batch of la confidential clones were 3 leafer's to begin with and transitioned into full fled 7 leafs as should. Have a feeling it's genetic related especially when taking clones, different genetic traits will show further down the genetic line. They didn't seem to be slow or unhealthy either.
Don't feel bad. My plants have never had 7 leaves. Mine only hae 5 including the little nubs near the stem. Probably just a genetic thing but whatever.
I think it depends alot on genetics of the clones your using etc , my last grow , all my plants had at minimum 9 leafers with a few having 11-12 leafers near the tops ( the 11-12 ers were on my Purple Nepals )
...and as a plant matures, each new node usually has another set of leaflets until it has reached it's genetic potential...but often with some variation. Cannabis is a dicotyleden and after those first smooth seed-leaves emerge, starts out with two opposed serrated leaves formed of one lobe each; the next set of leaves often have 2 or 3 leaflets, then 5, 7, 9...
I believe it is from stress....my plants normally put out 3 leafers when I transplant them from the cloner to the pot, it goes away after a few days and they then only produce 5, 7, 9 or 11 leafers.