If your downspouts connect to extensions that travel underground, they require occasional maintenance. Without it, they will eventually clog and it is almost always going to be due to either shingle gravel accumulating AND/OR a covered cap preventing anything from exiting.
First and foremost, every single line should have a clear exit point that can flow without resistance. The caps on these downspout extensions, even the good ones, are opened only by the gentle pressure of water flowing out after it's traveled horizontally a ways. There's very little force, and even a small patch of grass or layer of dirt can keep the cap from opening. Grated caps can be clogged simply by folded up leaves, right there under the cap. Once this happens, it's a short time before the cap is grown over entirely. If you don't know where your caps are, that's an issue.
The other issue is shingle gravel. This is one of those things you just cannot picture or really believe until you see it, but we have routinely left job sites with 40+ pounds of shingle gravel. Shingle gravel refers to the little granules that your roof sheds little by little every day. This rocky material does not float, and they accumulate in the gutters and drains – regardless of whether there are trees or not.
Every couple of years, or anytime there's a roof replacement, enough of this sediment builds to start restricting flow. Once the already weak water flow is reduced, debris accumulates faster. Soon, there's enough of this wet, sandy material for roots to grow through. This here is how the vast majority of truly destroyed lines started off – but it's salvageable until that pipe actually bursts from growth. Roots tend to enter from the ENDS of the pipes or at junctions and can sometimes, once loosened around the interior, be pulled out in one piece.
These two causes are responsible for the vast majority of problems we see, but there can of course be many other unique issues. As far as debris, we often find large clumps of nails or trimmings from flashing or shingles that were left during a roof installation and end up in the drains. Most larger objects, such as toys and fireworks, tend to get caught further up in the downspout elbow portion but we have found them in the drains as well on occasion. Light fixture hangers for Christmas lights are extremely common in these drains.
The lines can also be damaged – outright crushed or permanently restricted and in need of replacement or re-routing. They can also simply be out of slope. As we've covered, these lines tend not to have much of a dramatic slope, especially here in Indiana, where most of us don't live atop a hill – It's not hard for some of these to end up sloping subtly backwards, and especially in heavy rains, they can flow towards the house even when there's not an actual blockage in their path. Jetting won't directly fix these issues but it can help diagnose.
It is perfectly understandable to ignore these downspout drains. They are an afterthought. Doing so is a costly mistake, though. When these lines are proactively maintained, even with a cleaning just once every 3-5 years, they will last dramatically longer and perform much better. Don't wait until you get wet!