I find it very strange that you are unable to find Sigilyph in particular. Are you in the right area? Sigilyph cannot be found in the Entrance of the Desert Resort, but is rather found in the Desert part of the Desert Resort (i.e. the large area that is past the gate).
If you are in there, then there should be a 10% chance that you should find it. That's 1 in every 10 encounters, so it shouldn't be too impossible. The capture rates of the other Pokemon shouldn't affect that. The Repel trick wouldn't work either, as Sigilyph is only found strictly at Level 20, a level which all of the other Pokemon of the route can be found at. I'd say just to continue looking. They're there, deep in the bowels of the Desert Zone.
I don't have many good pointers on catching Sigilyph, but one of Sigilyph's abilities are Wonder Skin, which makes moves that only inflict status conditions work half the time, so keep that in mind (the other ability Sigilyph can have is Magic Guard, which only allows Sigilyph to take damage from direct attacks). Otherwise, use all the other conventional catching tips: weaken the Pokemon, use high capture rate balls, etc.
Hope I helped. :)