A couple possibilities:
Tabs-yes they work but many people have reported inconsistent results using them
DME-this is one method of bulk priming used however the process takes longer than using dextrose or priming sugar as it is a more complex sugar
The recommended minimum of time required to condition and carbonate bottled beer is 3 weeks at 70F. If the bottles are in an area colder than this then the process takes longer
For bulk priming with dextrose it is recommended to use .75-1oz of sugar to 1 cup of boiled water. Create a simple syrup, pour into bottling bucket and rack beer onto it. The swirl will stir it in for you as the beer racks.
It is best to WEIGH out the sugar as measuring by cup is less than exact and you can either under or over prime as a result so get a small kitchen scale, they're cheap but reliable
Have you checked your caps to be sure they are sealing properly and not leaking? Are you giving the bottles enough time to sit at the proper temperature? They just may need more time.
If the head is disappearing quickly it can be your glassware is just not clean. There may be dishwasher residue or rinse agent left on the glass and that will kill the head. Clean a glass with some hot soapy water and rinse well and pour, see if that problem goes away.
If you are using the proper amount of priming sugar, leaving the bottles for a minimum of 3 weeks at 70 and properly capping there is really no reason the bottles will not properly carbonate, you just need to be patient