Turn off all the auto-adjustments in your webcam's configuration software. OBS just calls whatever program is used to manage your webcam's settings. If it's set to auto-adjust, there's nothing OBS can do about that.
I had a similar problem with maintaining a good chromakey effect (with all auto-adjusts turned off) because I wasn't managing the light allowed into the studio; I grabbed a set of blackout shades for the windows and make sure I close the door when I'm casting during the day (so the only light coming in is from my lighting setup) and it works reliably now every time. Just make sure you let the lights warm up to operating temperature... especially if you're using CFL bulbs.