
Welcome Gonzalo!
I think I know which patterns you're talking about, but I've never looked at the SVG files. Based on the info you've provided, I might make a couple of guesses.
First, perhaps the line is grouped with other content. When you select it, look at the status bar (bottom of window in the middle). That will tell you what kind of object it is. If it's in a group, the status bar will say "Group of ....."
I suppose it might also be possible that it's a subpath of a larger compound path. If it's not a group, please tell us what the status bar says when you select it (with Selection tool

)
The other reason might be that it's either locked, or on a locked layer. If you're dealing with having something locked, you won't be able to select it at all. Look for the icon that looks like a tiny lock, beside the layer name. If the lock is open, it's unlocked. If it's closed, you can click on it to unlock it. If the object itself is locked....I'm actually not sure if there's any outward indication....and....hhmmm..... Well, if it's locked, you won't be able to select it, so I'm not sure how to unlock it. Maybe try Object menu > Unlock All.
If none of that helps, would you please provide the SVG file for us to look at?