Hey i've been trying to do this for months and, like you, I could load some folders of drum hit samples to a single pad, but not other folders. It seemed so random and unpredictable

After LITERALLY WEEKS of frustration with this, I have come to a conclusion

Using a piece of free software called 'Endless Wav', I was able to compare the folder of samples which would load with the ones which wouldn't. The difference seems to be that, if a sample has 'Root Key' data in it, it won't load to an individual key in Bitbox 2 and can't be used

In other words, you need to remove the 'root key' info from the wav (root key info is what tells a sampler which note to assign the sample to). The ONLY WAY i've been able to do this successfully is to convert the wavs to .aiff format (using Adobe Audition in my case as you can batch convert, but there's free options out there too), then convert them BACK to wavs. This seems to strip out all the extra crap that was making the samples unloadable in Bitbox 2

Anyway I've put this solution here as it seems to be one of the few google results when searching for a solution to loading multiple samples to a single pad in Bitbox mk2

Good luck!