Do you number the first and the last 6 or 8 bottles filled? I do this in case there is a carbonation problem. If after 4 or more weeks of warm conditioning the first bottle sampled is lacking carbonation and the last bottle filled is over carbonated I will know the problem is from the priming solution not mixing.
What temperature did you hold the bottles at for the first three weeks. Could be just a time thing or too cool of conditions for the yeast to work quickly. Higher ABV beers will take more time to fully carbonate than low ABV beers.
I wouldn’t add more sugar until time and temperature have been ruled out as the problem.
Edit:
Did you hear any CO2 hiss when you opened the bottles?
How long did you chill the bottles before opening? It can take a couple of days for the CO2 in the head space to carbonate the beer.
