After finishing the game, I noticed that the numerical values of the objects are not sufficient to reach the required upgrades. At some point, the object values drop back to 0, which prevents further progression.
Thanks for letting me know! Your gold probably also fluctuates between a really big positive number, and a really big negative number. If I expand the game, it won't be nearly as easy to break, so players won't run into the 0 value peg issue either.
