May be down to the choice of blades. I know they get a lot of praise, but personally I can't stand Derbys - I just don't get on with them even slightly. That'd be the first thing I'd change, apart from maybe the blade angle, to see if there's any improvement. There's a lot of variance between blades and it takes time to find the right one for you, but at least these blades don't cost the earth.
Next thing I'd look at is your prep and aftercare - making sure your face is clean, your lather is good and maybe consider using a pre-shave oil if you don't already, then afterwards I found an alum block really helped for me and a good moisturiser.

Sorry if this all sounds condescending in any way, but it's always useful to re-visit the basics if a problem arises.