From what little I know and I’m no expert, there are really two main types of prescribed burn processes.actually a few more but this is what most novices stick with. A backing fire, a fire into the wind, does a slow methodical burn, and burns up all of your litter layer, and a heading fire, one that burns WITH the wind and is faster, but may not burn your litter completely to the ground. It spreads so quick and much quicker with any kind of wind. If you dig into the ash of a heading fire, sometimes you will find the litter layer underneath that wasn’t burnt very well. It will slow the regrowth sometimes and basically act as a mulch in your flowerbed kind of effect. A backing fire is what we try to do most of the time as it burns slower and complete and our seed bank seems to spout and come to life. Maybe you could have done a heading fire, with some ground moisture on top of that and could have caused more litter right at the ground not to burn off. You wouldn’t hardly notice unless you dig down. It’ll all look good. That’s just my personal observations and my two cents. To really answer your question, you may need to go back in and do a backing fire through it if this wasn’t done the first go around. It could be other things that have been mentioned like poor soil or too much canopy and the seed didn’t receive enough sunlight. Good luck and let us know how things progress.

Here’s kind of an example I found. Fast forward to 2:00 https://m.youtube.com/watch?v=WVygOrWCoMI