Don't forget the bellows factor!

Also, you could be getting some short-exposure reciprocity failure; the flash duration may be 1/1000 or shorter.

I know your initial reaction is to buy a powerful flash, but in working with l arge format and tabletop subjects unless you spend a huge amount of money on a h igh-power pack you'll still be working with multiple pops, just not so many of t hem. By high-power I'm thinking of 2400 w/s or more.