There is a lot of talk here on self-sustainability. I'll try to couple two issues.
As already mentioned, it is quite hard for a high-tech colony to be self-sustainable. Once, because Earth would not want them this way (or else the colonies turn out to be competition in somewhat near future). This is a political issue, basically.
Once, because it's really damn hard to pull of, so most would not bother. Now, this is economy.
The issue is that we have with most resources is not that they are scarce. They are scarce at a given price, because somewhere else the is a mountain made entirely of iron ore; oil flows in fountains when you push a stick into the soil; a comet floats around, consisting of pure alcohol; deterium is 20 times more common in the ocean of that particular Jupiter moon; you get the point.
So, some resources are replaceble, but at a much higher price. The price might be so high, that no one thought of mining them at that colony. The price might be so high, no one thought of providing means to mine them (or means to produce means to mine them) to the colony. In the pre-disaster, perfect Sol-globalistic world it was not needed.
To add insult to injury, some resources are really scarce and are produced in a very few places. Those are the first ones you loose as a "lost colony". The spice is only produced at Arrakis, as we all know.
The colony might not have the means to mine and process all kinds of resources it needs. This means that the moment it does not have a connection to the mother world, the colony can only rely on things it can do itself.
To put it short, the technological level drops.
Because they cannot produce the current means of production (or some iterations of it, they cannot produce the means to produce the current means of production, and so on), they are limited by the means they have and can produce. These would be more outdated.
Some tools are now unique artefacts. They work until they break. There is no way to produce a new such tool. In some cases the colonists know how, just cannot pull it off. In some cases they don't even have the understanding, not only the technology.
(Try building a modern processor – or any integrated circuit actually – with the top-notch technology from 1920s, for example.)
And now the next problem. The tech-level drops. But the natural environment of the colony cannot sustain Earth-based life as it. People (and all the Earth ecosystem on the colony) need life support. Life support is also technology. Did I mention, that tech-level drops?
So, at some point, the pre-catastrophe, advanced life support is beyond repair. Probably, some kind of a more primitive life support system can still be built. It would probably support less people or in a worse manner. This downgrade of a life support can happen multiple times.
I can actually image one of the latest space-faring trips of the colony to get some particular rare Earths or some particular carbon nano-tubes or some tritium from nearby asteroids – as long as they can. Because developing mining / local geology / nuclear industry takes too long and they cannot wait, so it's easy to trade one unrenewable resource (space travel) for another, also currently unrenewable, but much sorely needed resource. This is a prolongation of an agony, if they are not able to contact and trade with other colonies. If they can, space travel suddenly becomes much more important. (But tech level decrease bites here as hard as with life support.)
If the colony is caught in a downwards technological spiral (tailspin, actually) and cannot rely and re-base their tech to the currently available materials and resources and still manage to keep some kind of a life support running, then the colony dies out.
This is basically one of those "not with a bang, but with a whimper" stories.