I keep on getting ads not shown due to the low ad rank and wondering how to increase this. any ideas?
If your seeing this in the bubble next to your keywords it relates to all traffic, so if you are bidding +300% on mobile you should be showing on mobile devices but not on desktop/tablet.
Instead of relying on this message you should check the ad preview tool and look to see if your showing on mobile devices.
If your ads still aren't showing there are two things you can do to improve ad rank, 1.) improve quality score, or 2.) raise your bids, these are the two factors that determine ad rank, essentially ad rank is QS X Bid.
Example:
a.) QS-4 X $2 = AR-8
b.) QS-9 X $1 = AR-9
So in the example above b is bidding half of a but due to a higher quality score it will have a better ad rank and appear in a better position