My Shark has a blue push switch at the front of the handle just below the top handle which you can move left to right to change the level of suction

If it helps others I finally sorted the problem. Although I had checked the flexible hose was clear I hadn’t checked the handle itself which I found was completely clogged up. Had to use a long knitting needle to gradually clear it out. Now restored to full working order.

My shark lift away is leaving a faint line of dust either side of the brush. It is picking up dirt and bits ok. I’ve cleaned and reassembled the filters but that has not lead to an improvement. The dust collection tank is empty. Any ideas as to what might be wrong?