Could it be because I waited for the glue to dry 30 min before inserting the biscuits?
Yes, very much so! When using all types of PVA glues (white or yellow) you glue and assemble expeditiously.
Should I have inserted them when I first applied the glue?
Yes, it's standard that you glue and assemble in one continuous operation and without delay.
This isn't just to prevent an issue like this where it appears partially dried glue may be holding the joint apart in places, it's also vital for maximum strength1.
Your photo also seems to indicate that you're using too much glue. Now just to be clear, an excess of glue is a good thing, but the amount to aim for is just enough that squeeze-out shows consistently along the joint and not for large amounts of glue to drip out. You'll eventually get a feel for the amount that's in the Goldilocks zone, but don't sweat it if you overshoot the mark every now and then (it's much better than the opposite which can indicate the joint is under-glued).
One additional point if needed
I'm adding this for future readers as much as yourself, biscuits don't add strength to a joint like this.
In a long edge joint like this, which if done right glues together very strongly2, biscuits (as well as splines, dowels or Dominos) serve as alignment aids only. But if your glue-up strategy is good, so that board alignment is assured e.g. by using cauls or a clamping jig, they become superfluous and you can keep them for other joints where they do add strength, such as a 90° butt joint.
1 See the bullet points here for a quick summary of what makes for a good glue joint. And a little on the likely causes of joint failures in the second footnote here.
2 Literally creating a joint stronger than the wood around it, even in much tougher woods than pine or fir.