What Causes Hair And Scalp To Be Oily And Smelly ?

Recently (the last 2-3 weeks) my scalp and hair have become very oily and has started to smell a bit like old cheese after a long day when my hair is more oily. I shampoo everyday now, and have tried various different ones, but none seem to work.
What's going on, and what can i do?
n.b. I have had scalp psoriasis for 15 years that ebbs and flows in intensity, and right now it is not that bad, and I am not treating it as there is nearly no need.