If a beer refuses to form and then hold a good head, it can be either the beer is under gassed (more prominent in a keg situation) or insufficiently matured. Beer that has had longer to mature means the alcohol and the C02 have had longer to bond, the Co2 molecules are now smaller which take longer to float to the surface of your glass of beer and pop, in turn the head holds longer!
Corn Syrup (or moltodextrin) will help with achieving a better beer head, usually add 250-500gr per 23 litre brew at the beginning of fermentation (add with tin of beer concentrate and additional brewing sugars).
Note: Beer glasses that have been washed with dishwashing detergent or put through a dishwasher will seldom hold a head as the detergent puts a film onto the glass.