I'm running, 4, QB304 on the same HLG320H-C2800 and it certainly isn't running "soft" IMO! And I've wired in parallel so each board is running at "only" about 85 watts per board. The 320H-C2800 should only be used for parallel strings, IMO, as it exceeds the amperage for a single board, but lacks the voltage for 2 in series. It really pays to learn to use the manufacturers spec sheet for these drivers, they tell you this information. Here's a screenshot of the C2800 specs:
View attachment 3969708
This tells you the operating voltage range for this driver is 57-114 volts. Thats only enough for one QB304 or 2 QB 288. BUT at rated current of 2800 ma, far exceeds the QB304 max rating of about 1.6 amps ( 1600ma ). However, wired in parallel, you can run multiple board: 2, 3, or 4 as when wired in parallel, the boards share the amperage as it is divided equally among them. So, 2 boards would run @1400ma, 3 @ 933ma, etc. So my 4 boards ( and probably yours ) run at 700ma per board. The voltage remains constant, in the 104-110volt range as per actual draw.
If yours appear to be running soft, do you have a dimmer? If so, try disconnecting it and see if thats better. I rum mine wide open with no dimmer.