I think TS only has the Holga 135BC at the moment, and it doesn't have metering or any adjustments to shutter and aperture.. but still 6 rolls of 200 film shot in daylight with nothing much turning out is still a mystery..
To put it straight forward: you have the wrong tool. You choose a manual car with only two gears and no instruments but you only have watched your father in this automatic car with 2 pedals and a gear lever that he uses somehow. And you're cursing the man at the petrol station that your car is too slow.
Go to your library and get some decent books about the very basics of photography. You might even want to start with any cheap digital PnS that can be put into Manual mode so that you see the result immediately. This way you'll learn faster about aperture, ISO, shutter speed and also about metering. Once you know this you can return to your Holga and I'm sure your pictures turn out much better.
How much more likely is it that the source of the problem lies with the party who doesn't know much about producing good images, as opposed to the labs who know how to go about their livelihood and have done it for years?
It's like a guitarist blaming his guitar when he plays wrong chords.
So was it underexposed? Or were all the films UNexposed?
If unexposed ah, I think maybe the reel didn't catch the film properly, so actually you didn't manage to expose any of the frames. I've done that before, once, and I won't ever do it again. Make sure to rewind the film abit after loading to ensure that it's taut in the camera. That way, you know that there is a sure catch on the film sprockets.
If it's underexposed, maybe try using 400 film instead of 200? Although I'm quite sure 200, and even 100 film should expose pretty ok on a sunny day.
I hope you didn't get the Velvia as yet. Like others said, slides are much less forgiving and have a lower latitude/tolerance for wrong exposures. But having said that, you could try a Velvia 100 then go cross-process it. The uncle at Ruby photo tells me that cross-processing usually kinda increases the exposure, coz when I took properly exposed slides to cross-process, the results were really blown highlights... like overexposure. Not sure how true that is though.
Hope you get to enjoy your 135BC... =)
2nd thing, shooting Holga is all about fun, and the fun about producing unpredictable results, but also need some basic knowledge about photography
3rd thing, color slide cost more than negative, the cost or processing too, do you want to spend some monty just to try your luck? or should learn some fundamental photography first?
4th thing, don't forget the lab have process countless film and doing this for living, tho some time some labs do screw up, but for your case is very unlikely, you need to find some other better excuse.