I use AP all the time and never had this issue in the past so I assume its a preference the retailer can choose?
yes that's correct it's reatiler preference. some will automatically disable the contactless feature if it's over £30, some still have contactless stopped at £20 (though i've only seen this at one retailer).
if it's over £30, on barclays at least you have to tap the phone to the reader, it then buzzes you to type in your
card pin (not phone pin) into the phone (you can move the phone away from the reader at this point) and then tap the reader again within the next 30 seconds. this allows over £30 up to £100, but as said, some retailers just simply decide to not allow anything more then £30 so it doesn't always work, though rare.
another issue you sometimes have is although the retailer may allow up to £100 contactless with mobile, when you first tap the phone against the reader and it then asks you to type in your pin of the card, the reader sees this as a fail so then prevents contactless or needs to go back to the person on the till to select the card payment/contactless payment option again for the reader to reanlbe contactless, so you might not then be able to pay because of that reason and not the phone.
to solve that issues, i'm not sure how AP works but barclays you can login and get the contactless limit up to £100 ready before you get to the reader.
to to sum it up, it's preference of the retailer. most do allow the £100 payment, but almost no staff know this and will tell you to pay by card. it's quite fun to see their faces when you can make it work.