When I was trying to pay for my parking at the Highcross Shopping Centre, Leicester my card was declined twice. The amount was £2.80. With my HSBC card this usually appears on my statement as UPT - Highcross LE Southampton. The failure also isn’t shown in the Mondo app.
The parking machine’s card reader quite possibly operates offline. This alone shouldn’t cause it to fail, but would prevent the Mondo app from showing the failure.
The machine may have rejected your card based on IIN/BIN - Mondo cards usually appear as prepaid, which many companies don’t like.
That’s annoying, sorry! Parking machines tend to be offline as @billinghamj mentioned which can cause issues with the prepaid card 
A Peak National Park cashless parking machine rejected my Monzo card this week but accepted an alternative (debit) card.
It seems an odd position for the vendor to take on prepaid cards given the higher probability for tourists to be using them.
Wow, they have finally turned them on after being installed for the last year!!
Back on topic, the decline is possibly down to the transaction timing out as these machines use GPRS for connectivity so if a response isn’t recieved in a given period it will decline the transaction for online only cards