Thank all for the feedback.
The wood arrows I purchased from HHA came with 11/32 Bohning Classic Index nocks, which were indeed too large for the serving on the 10 strand flemish which came on the Big 5. But that was easily rectified by dipping them in hot water and then pinching them smaller.
The real issue for me is just not having enough strands to control the stretching on my Big 5 longbow. I feel that for B50, it is too much force spread over too few strands, resulting in more stretch per strand.
For example, if I set my Big 5 brace height to 6 5/8", after shooting for a while it will be 6 7/16" or less. Then, after having it unstrung for even a short time, when first stringing up it will be 6 5/8 again.
When I first got my two recurves they came with 12 strand B50's. I now use 14 on my 40# recurve, and 16 on my 50# recurve, and they are both quite stable, as well as quieter than a 12 strand.