How strong does the agar have to be?
 

I've only used premade media with agar already included. But from what I remember from discussions, 7 g/l is about standard (+ or -). I may be wrong here.

I doubt the stiffness of media would affect germination, but I may be wrong here too. :) I have had a few batches that didn't gel like I wanted them to, and a few where it was too stiff. I attributed that to poor mixing while dispensing and/or not fully dissolving all of the agar. Since I've made it a point to get it good and hot and stir frequently while dispensing, I haven't had another problem.

There are different grades (purity) of agar. Where did you get it?

My only guess is that it didn't get fully dissolved or was a bad batch. Again, I may be way off on the strength but that's my recollection.

Did any of the flasks gel? Did a portion of each (like a layer at the bottom) gel?

I have used agar at 6 gr per liter with good success...
I use premade media now!

Yeah, from 6 to 8 gr per liter is good enough to get a thick medium. Maybe your problem is the heating.

Are you autoclaving the entire batch in one vessel, or dispensing then autoclaving? If the medium doesn't get hot enough to fully dissolve the agar, it gets unevenly dispensed into the flasks. You wind up with a super thick layer at the bottom of the last few flasks and the rest are watery.