How do I create "solveable" random levels for a sidescoller?

First of all this is my first post here, after reading a lot of threads that always helped me. Thank you for being such a helpful community!

I am starting to create a sidescroller in libgdx, that has a fixed position for the character on the left and scrolls the background to the left, so you get a feeling of flying. You can freely move up and down with a set speed. The difficulty in playing lays in a bar that stands for some sort of "power" and decreases over time. You need to collect certain items to fill it and others decrease the bar (so they must be avoided by all means).

I am currently keeping them in a stage and position them randomly, but this is not a perfect solution, as the case sometimes appears, that you even die when collecting all items, by a simple lack of items. How can I create my levels with a certain logic in its randomness without making it too easy? Also, I later on want to create more obstacles, which don't block the access to the "good" items, but make them difficult to reach. How can I generate a level with a fixed length, composed of good and bad items that make it hard, but possible to succeed?