Firstly, I would suggest that the claim approval/rejection rates are based on historical data..... so just like the share-trading adverts, these are not fixed and can go up or down. They are there to give you an indication of likelihood, they are not bible truth.
Secondly, imutual, like all cashback sites, only pays you when the merchant or their agent send the money to imutual.... and this does NOT always happen. If the merchant starts to develop a pattern of non-payment (amongst other things) then they may be suspended (you'll have seen that), but that doesn't help you.
So, imutual instituted a unique cashback guarantee scheme to overcome this issue and, if the merchant is covered by the guarantee and doesn't pay, imutual will pay you itself. Raise a query and ask.
However, there are still merchants who are not covered by this guarantee..... and you have to ask yourself why not - and act accordingly.
Please be aware that I am just a member, like you - so the above is my personal view, not an official response. To get the latter, raise a specific query using
https://www.imutual.co.uk/queries