If they aren't ready after 3.5 weeks @ 70 degrees...
Then they need more time...I have had beers take 8 weeks to carb and condition...It's a natural process...so it has it's own agenda based on the OG of the beer AND the temp of the bottles...It is ACTUALLY theoretically possible to
not add any priming sugar and with time to have the beer carb up (for lower grab beer like yours) In fact old english brewing books have recipes that DON'T add sugar...I ran some of the recipes though beersmith, and that corroborated that fact.
You just need more time, that's all I never worry unless it's 8 weeks for a normal grav beer..heck my 1.090 OG Belgian strong took 3
MONTHS to carb....
Read this;
Revvy's Blog, Of Patience and Bottle Conditioning.
Lazy Llama diagramed it best...
But 3,5 weeks is still WAAAY within my not worry zone...especially if the beer is below 70 degrees.