I had the same problem. I started dosing into the HOB until some undissolved particles got into the impeller and made my HOB quit working. Thankfully I was able to get it cleaned out and working again.
Then started using a jar to pre-mix but even so still can get GH booster to dissolve. Can't get it to dissolve even in boiling water. Now, the day before I need to add it, I boil water, put my GH booster in, add that to a 1-1/2 gal jug and shake, stir, shake, shake and every time I walk by it shake it again. Still wasn't dissolve the next day but at least the particles were smaller and poured them directly onto the sponge in my HOB so they hopefully won't get into the impeller again. Plus I needed to top off my tank so got 2 things done at once. I'm really getting aggravated with the GH booster. My tank is coldwater tank so the GH booster stays in the tank for 5-6 hrs before it's finally totally dissolved.
The other ferts I don't have as much problem with. Use the jar and they dissolve MUCH quicker.