It sounds like you have a classic stuck brew: brews ferment down to 1.020 then stop, rather than getting down to the usual 1.010 or lower.
Can be hard to pinpoint the cause but some reckon it's due to lack of yeast. Did you happen to notice how big the yeast packet that came with the kit was? Some kits provide a small 6g packet rather than the usual 11/12g packet needed for a 23L brew. Having said that, I've just had an AG stout stick at 1.020 with a full-size yeast packet.
