HI Gary,
Welcome to the forum! This type of question should really be posted to PowWow, as the Shooters Form Forum is limited to questions about shooting form, and not equipment related issues.
That said, I'm really afraid I'm going to confuse you more than help you, because my typical results are so different from yours. I draw 28", and also shoot a 40# longbow, among other bows. My longbows are cut past center, whereas yours is cut to center, with a rather thick 1/8" strike plate, which means it's really not cut to center. Your 1/2" longer draw should indicate a slightly stiffer arrow than mine, while your less than center shot should indicate a slightly weaker arrow than mine. I'm not sure the two exactly cancel out, but my results should be in your ballpark.
With your setup, I would be shooting a 600 spine arrow (or a 1535) with a 100 grain point, or a 500 spine arrow (or a 3555) with a 150 grain point. Or with wood arrows, I would be shooting a 45-50 spine arrow with either a 100 or 125 grain point, whichever shot best. So I would definitely not be getting a weak indication with your setup. I'm not sure why you are, but there are form errors that can cause false weak indications. So if you would just like to keep shooting your 3555's with 125 grain points for a while, and maybe try a 145 grain point to see if it works better, or wood arrows in the 45-55# spine range, I'm sure they will work fine until you can fine tune your arrow selection down the road.