Four weeks is long. I bottle condition in the 50s-60s, and it's never taken four weeks for any of the beers that I have brewed. Take two of the beers from random locations in your beer stock pile (box, shelf, whatever). Put each beer in the fridge for two days then crack them open and report back what the carb levels are.
My thought - something went wrong...what was the environment where they were being stored? A fridge? If so, okay...that makes sense. If not, perhaps your sugar solution was not mixed well enough into the batch and instead some are carbonated and some are not.
Maybe something else went wrong - can you detail your brewing and bottling process?
And here's my pitch : )
Instead of batch priming you might consider bottle priming in the future. It's very simple and it insures more accurate volumes of carbonation for each bottle! Regardless how well your mixing process is when batch priming, you will also get a more accurate volume of carbonation by bottle priming and not batch priming.
Take your sanitized bottles and put a sanitize funnel on top. Pour in 1/2tsp of white table sugar into each of the 12 ounce bottles (adjust volume of sugar based on your bottle choice) until they all have sugar. Then get your beer into each of the bottles however you're doing it (bottle bucket, for example), cap and wait.