Thanks so much for playing and for the detailed notes!
You're right, this part wasn't clear enough. Rolling wears your rock down a little each time (that's the cost of a roll), and the ore you bring back buys upgrades. The Snail needs about 23g, so keep tapping (or let Auto Growth work) until the "Next" bar under your rock fills up, then roll again.
The game did show that bar and a few suggested upgrades, but they were easy to miss, so I just pushed an update: when something blocks your roll, the results screen now shows exactly how much weight you need (for example "Weight needed 23g, now 9.2g") and suggests the growth upgrades. Thanks for the great suggestion!