I think that enlarger head has two lamps in it. The lamps get dim over time. The dicro mirrors get dirty. That affects quantity of light, but little else. You should keep a spare lamp; that'll tell you what you need to re-order.
You may need to clean the lens to increase contrast; make sure it's not hazy. You may need some extra contrast. Maybe the film carrier slot is leaking light into the darkroom and fogging paper during exposure? Lotsa possibilities. Maybe your developer went weak over the few weeks since you did the contact prints?
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