Sometimes when we do Tummy Time Dylan will lift his head up really well but I have to pick the right time to do it! If he's tired he'll just lie there sucking the mat, and if he's grumpy he'll just yell into the mat!
Try making sure all the interesting stuff (ie you and his toys) are in front of him, otherwise its too easy just to look to the side. I also hold noisy toys high in front of him so he lifts his head to look. Good luck, but don't stress - all babies develop at different rates!