Line sizes are based on the AFTMA Standards, which are a function of the weight of the first 30' of line. for example, the first 30' of a 6 wt line weighs appx. 160 grains, and they change by 20-25 gr. per size (5wt is appx. 140 gr, 7wt is 185gr, etc.). The rods are labeled such to be able to adequately load and unload (assuming that the cast is properly made) on this basis. There is probably one of two things going on in your situation.
The first is, as you proposed, that the line is too light (undersized). This can be easy to check by casting a longer length of line, say 40 - 45'. That will provide the additional needed weight. Normally you would lengthen you casting stroke to accommodate this addition line (longer casts require a longer casting stroke, lest a tailing loop), but in this experiment, you want try different line lengths using the same stroke length. In effect, you are changing the length of the line until you fund the weight of line that the rod is designed for. If it corrects the problem than that's your answer - go to a heavier line. You could mark that length of line (from what's outside the rod tip, to the end of the forward taper, which is usually about 5' from the end of the line) and take it to your local fly shop and ask them to weigh it for you. Umpqua make a line scale just for that purpose and it cost about $20. It's a handy thing to have if you mess with lines at all. However, if you buy the new line form them they will probably weigh it as a free service. btw, I agree with those who suggest a double taper, especially for roll casting. Here is a link to chart of the fly line sizes and their respective weights.
AFTMA Fly Line Size-Wt ChartThe second possibility may have to do with your casting style. If you are not accustomed to casting a slower, rod, i.e. a Heddon Black Beauty, then there's a chance that you are not stopping the rod with a positive stop. This takes a little practice with a rod as different from graphite, as a 9' Heddon Black Beauty. An indicator of this problem may show up in your back cast. You didn't mention weather , or not, you get full turn over in the back cast. If you are, then it's a problem in your forward stroke (stop). If you are not getting a full turn over in the back cast, then you
will have slack in your line when you pull through the forward stroke, depriving the line the energy required to turn over properly.
This is sort of difficult to diagnose without seeing the cast, but if you have a certified casting instructor in your area, they can help you identify the issue. There is link to a directory of all FFF CCIs on the FFF website
FFF-CCI Website. Or, if you can video your cast, I'll gladly look at it tell you what I think. Otherwise, come fishing on the White River in Cotter, (North Central Arkansas), where I live, and we can fish, cast, and talk "bamboo".
Also, Look to see if there is a designation such as HDH, or D, or some other letter-type marking. This is the old standard line size that refers to line dia. and is often found on rods that were made during the silk line era. H line refers to .025" dia, D = .045, thus, HDH is a double taper line that tapers from .025" to .045", then back down to .025". There's no exact correlation form the old system to the new because the modern lines aren't all the same diameter for any given weight. But, you can get close enough to at least find a starting point to do the test I suggested, above.
I hope this helps,
Denis Dunderdale