Operator error for sure Cuppa. Sorry mate. I understand your frustration because i have seen an exhaust jack not cooperate until it is positioned well and the inflation works perfectly if done right. If done wrong it will struggle to inflate, be slow and leak.
The cone on the end of the hose will cater for the 3" exhaust no problem but will really struggle if the and of the exhaust is not cut off square. If there is an angle forget it.
Yes you need to lean in and push it on hard. Yes you need Mrs T to Rev the car! Forget trying to lift your car with your engine idling away. Zero chance of any more than slight lift off idle.
When i worked at the Avalon Airshow last year Tarmac Ops had 10 exhaust jacks that they had in the Store but had never tested. They asked us to unpack and test them. I had no trouble getting full lift out of every one.
The best kit i have.
Dont over think it. The bag does not need to be neat and straight. Actually it will probably never be. It needs to be well placed and stable but that dosent mean uncreased with straight sides.
Best place to lift is under the diff housing. Have another go. Or bring it to me and id be happy to show you. Or google on youtube.