Have you tried taking the keys off and looking underneath them there may be stuff stuck under them that is not allowing the key to be fully depressed. Happens a lot when your kids eat biscuits and chew lolly sticks you would be surprised what i have found. i normally turn keyboard over and use a paint brush to get in between the keys, surprising how much stuff comes out..
Thanks for all the responses so far. I will give the keyboard a good shake and clean especially around the problem keys. If that fails I will probably try a new keyboard. Didn't realise they were so cheap.