That is Ad data. SEO has no effect on ad positions

It's likely that the desktop bids for that keyword/group are higher than the mobile bids. So if you are bidding the same on mobile and desktop then your desktop ads may show less and/or at lower positions. You should probably increase your desktop bid and decrease your mobile/tablet bid.
Of course that will depend on what your conversions are like for each platform. If mobiles are converting well then maybe not reduce the bid. CTR will likely suffer as well.

You need to look at all the targeting factors including geo/demographics/platform/bid price to fully assess what needs doing.