How long do you condition for? How are you adding the sugar? How much sugar? What kind of sugar? What are your conditioning temps? How are you capping your bottles or are you using the Mr. Beer bottles? Need more info.
How much sugar per bottle? If you are adding sugar by the bottle you can have inconsistent priming. Your temps are good. Three weeks at 70 degrees if the minimum is primed correctly. Do a search on batch priming. This is a more effective method to ensure an even amount of sugar in each bottle.