If its taken enough water (can't remember exact amount) and definitely hasn't got any air locks present, then it sounds as though it could be blocked.. First try draining it out again, and flushing the matrix inlet + outlet pipes, plus top + bottom rad pipes etc with a powerful hose, until they run clear, then re-fill and re-run-up. I use a 2 litre bottle to fill mine so I can count the water in, think its 4/5 litres ish. Think there's a guide on here advising about the running up process, cap off, engine on idle filling bit by bit etc, but I never have probs just adding water + squeezing pipes etc with engine off, until it takes no more, then adding cap, running to temp and adding extra if required
