Have you considered that your cards may not be genuine, but may be
fakes?
Far too many
memory cards sold on the internet are fakes, which are typically a lot slower than genuine cards, and/or may be lower capacity than what they are supposed to be.
If you are buying any
memory cards online, be sure to only buy from authorised resellers (for example,
1800memory is an authorised SanDisk reseller on eBay, but many other "SanDisk" items on eBay are fakes).