What nutes/soil are you using, and what is the ph of your water?
If the ph is too high, it will lock calcium and magnesium out of your plant. If you aren't giving your plant any CalMag at all, you just need to start supplementing.
Edit: also get a scope and check the bottom sides of your leaves just to make sure no critters are sucking the life juice out of your leaves. Some pests cause rust colored spots like that as well.