Many users in Sri Lanka seem to be having problems with bank cards lately -- or at least many are posting similar comments. You may want to contact your bank to see if there are any new restrictions or regulations regarding using your bank card on eBay.
If you can figure out what the transactions that succeed have in common, and what the transactions that fail have in common, and what separates the two, that might be helpful in figuring out where the problem lies, and whether it is a technical issue or a legal one.
It is unlikely that users in the US or other countries will be able to provide much help, since this particular issue seems to be a problem in Sri Lanka only.
