Payments through an agent or third party
It's not unusual for a business taking payment to be acting as an agent for the actual supplier. In this circumstance, Section 75 may not apply and you may not be able to claim against the credit card provider.
A good example of this is when you buy concert tickets. If you buy direct from the venue, then Section 75 may apply if the cost is over £100. If you buy through a ticket agency, then it may not.
This is because the card provider may argue that as payment wasn't made directly to the supplier of the goods or service, Section 75 doesn't apply.
So where possible, make any credit card payment direct to the company actually supplying the goods or services.