I’ve also completely forgotten to shake it for two hours and then come back and shaken the bottle for a minute and then again 10 minutes later for 30 seconds. No matter how you do it, the HA powder does eventually dissolve and it makes a gel. But you have to make sure you keep shaking it regularly or it gets jello-y in places.
Every time I use the HA serum I made, I take it out of the fridge and I shake it for at least a minute. I also use the dropper on the glass bottle to suck the mixture up and down and help make it consistently gel like.