I kind of agree with Tom here.
What was the volume; width, and length of your previous board.
I do not weigh 100 lg. (220.5 Lbs) so I cannot tell from first hand experience, but
I weigh 75-80 Kg and can comfortably uphaul 90-100 liter boards.
Perhaps your rig is quite heavy (mine tend to be very light, even in the larger sizes as I use 100% carbon masts and carbon booms in the larger rigs) and this is somehow adding enough weight to put you over the balance point between weight and volume.
Perhaps its a little bit of a technique (or adjustment) situation.
What part of your board is sinking? The tail.... The nose... or the whole board goes down level?
Are you sailing in freshwater or saltwater. The difference in buoyancy could make a significant difference.
Often with smaller lower volume boards the sailor has to make some technique adjustments to get their weight centered over the fore and aft center of flotation of the board (which may be in a different place than on your previous boards.
Do you put your front foot in front of the mast when uphauling?
If you are "centered" over the mast foot on your Futura 122 and the nose sinks, move your back foot back a little and slide your front foot back closer to the mast foot.
If, as you pull your rig up out of the water the fore/ aft balance is changing, you may need to change your technique to shift your weight a little to the rear as the weight of the rig is applied to the mast foot.
I agree, you are right near the "sinker" point, but there may be some things you can do with technique that will enable you to comfortably uphaul the Futura 122 that are a bit different that what you were accustomed to doing on your older board.
And, is the board simply sinking to the bottom when you beach start or is it simply sinking a little until most of the deck is awash?
Many sailors comfortably ride boards that float them, but most of the deck is underwater before they start moving.
Hope this helps,