It would help if you tell us if you're on PC or on console, and if on PC if you're using the AwesomeGuildstore addon
If you do use that addon, try to reset any settings within that window; I've had trouble with this a few months ago where I couldn't see ANY items in any store, but after resetting everything, they reappeared.
Also, Solvents are not that many that it would be difficult to see the Reagents inbetween, so if it still doesn't work, just scroll a bit more

I get that as well (with and without addons). I just go back to 'provisioning' tab and select something else, such as enchanting, blacksmithing, doesnt matter, then switch back to provisioning and the box reappears. This is what usually works for me anyway..