I have a zz plant doing the same thing right now, and yes, in my experience, they do take quite some time to unfurl and mature. I don't know how long it takes exactly, but i have had two pushing from the ground for at least a month now and they are just now beginning to open up at the tips.
unless someone with more experience than i've got wants to chime in here i would say to keep on doing what you're doing!!

AlexLeo,
I also agree that the unfurling and maturing takes awhile, so don't be worried about that. I am concerned about the leaves turning yellow though, I believe that's a sign of overwatering. Every two weeks sounds a bit too much imo. I have mine in a 10'' pot and I water it about once a month. Hopefully someone with more experience than myself will chime in as well.

Planto

When mine turned yellow it was because of overwatering. It was in the gritty mix and I had been told it wasn't possible, so I discarded the thought. It's gone to ZZ heaven now.

Hi Alex,

I believe it's normal what's going on w/ your ZZ plants; the new growth does take some time to unfurl. Both of mine are doing the same thing now too, each of 2 pots I grow have 3 new sprouts. Generally they seem to put new growth like this in Sept. & again in Feb. from what I've observed in prior yrs.

Each of mine has had a yellowing branch in the central part of their pot, I believe that to be natural die-off of the older growth. If you give the yellowing growth a gentle tug, it may come right off in your hand as mine did. Should you see a lot more of that, I'd be concerned, if it's just a branch or 2, I wouldn't worry. Maybe cut back watering to every 3rd week? Also are they in clay or plastic pots? Any chance of a picture?

From the sound of it, it's all normal behavior so I'd say keep doing what you've been doing.