If we drop you in the middle of a pristine wilderness with an AK-47 (or even a simple matchlock firearm) and told you to build a copy of it utilizing simple stone age tools and you fail (because you will), is your failure because you do not comprehend or understand the technology involved?
Both. My understanding of how firearms works is limited to pull the trigger, hammer hits gunpowder(I think?), death comes out of the barrel. Give me a gun smithy and you'll end up with a pool of slag, melted plastic and burned wood.
If we gave you three damaged AK-47s (with only enough functional parts between the three to produce one functional firearm), in a pristine wilderness, with simple stone age tools, you could probably (through trial and error) produce a functional firearm.
With stone age tools? I doubt it unless me bashing an AK with a semi-sharpened rock will magically disassemble a rifle held together with screws, bolts and other interlocking parts. Even a musket requires some specialized tools to maintain, let alone build.
There is no reason to suspect that Jawas are incapable of innovation. There is no reason to suspect that the Jawas do not possess a thorough and complete understanding of the technology they work with and work on.
They putter around a failed colony world in ancient steam-powered crawlers they didn't build and have mostly confined themselves - voluntarily or not - to the Star Wars equivalent to a sleepy little farm town. They collect shiny refuse and sell it to locals. At best, they know how to identify a usable piece of scrap or machinery (Is it rusted or not? Does it go?), a skill the average 19th century tinkerer would have. They've had contact with space-faring extra-terrestrials for at least 4000 years, and yet have not been shown to have ships of their own or even an interest in the affairs of the greater galaxy at large (see also Tusken Raiders). Unless they have some sort of cultural aversion to technology that would actually make their lives easier, I'm not seeing how they grok what they work on except at the most instinctual level or through simple trial-and-error.
However, given that they are scavengers and that the moisture farmers (and others) on Tatooine rely on them to supply parts and technology, it tells us that both the Jawas and the moisture farmers lack the logistical and population base necessary to manufacture new technology and that, in many ways, the Jawas are more capable than their client base.
If the Jawas are able to scavenge parts that the farmers can use, there's no reason for farmers to manufacture their own. Again, all that tells us is that the Jawas have - through trial and error - been able to identify stuff that can be sold, and stuff that can be left in the sand. That doesn't mean they get how it works, they just know what the people want.