i don't know whether this would be at all related, but if i turn off the engine when the cruise control is switched on, the next time i start the engine, the cruise control won't turn on. the green dash lamp won't turn on either. so if it is switched of, when i turn off the engine, the next time it functions ok. maybe it is a design issue.
anyways, otherwise, while it is running, just a quick push of the speed memorize button would do the trick for me, no need for three seconds. i remember that it failed to engage at times when i did that 3 sec thingy. (although the manual suggests so)
try that way, maybe? : switch it on, when you achieve the speed, just turn up the memorize button quickly.