Huehnerschaender,

Occasionally, you'll encounter a level that can not be completed. The occasional few might not have enough trees to give you the height you need. The levels are randomly generated, so try another game at the same level.

Almost all landscapes can be completed; you just need to make clever use of energy, or look carefully at the opportunities presented by the landscape. For example, if you're standing on a boulder, its two units of energy are committed to that boulder. Sometimes one large step is better than two small steps. I'll leave the rest for you to figure out.