Here are a couple of guesses, though the thing that puzzles me a bit is your saying your stunted foxglove has unimpressive flowers. There IS a dwarf foxglove, namely Digitalis obscura, but but even being fall smaller, the blooms ARE quite showy even though the plant is very small compared to our native Foxglove. Here's a page about it:
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large foxgloves are biennials & unless they are self-seeding, your statement that they've been small & unimpressive for "years" suggests it just isn't the species you thought. D. obscura by comparison lives for several years.
Yet another dwarf foxglove is Digitalis dubia, but this one looks much more greatly like the full-sized Digitalis purpuraea, merely scaled down to an eency size. It's showy as the all get-out despite its tininess.
You may just have the plant misidentified altogether. Penstemons can sometimes look rather like foxgloves, but never as showy; one species is even named for the resemblance, Penstemon digitalis, but all the other species also have thimble or bellshaped blooms. Penstemons have extremely pleasing flowers but if one expected foxgloves, they might look shrunken & unimpressive by comparison, plus penstemons can "wear out" in time, or in stressful conditions, & thus produce fewer & much less impressive flowers, or the blooms might be very nice but get lost in an excess of foliage.
It might help to know the color of yours. D. obscura would be yellow or bronze; D. dubia would be the same bright rosey color of large biennial foxgloves, Penstemon digitalis would most likely be white to very pale pink, but penstemons in general come in all sorts of colors.
-paghat the ratgirl