Hi Oknodian, I've patched the issue with the fuel now, the game should be able to access all 6 levels and land on the moon.
Died on the copper rocket level, so I don't know if the bug affects levels afterwards.
Basically, during stone and copper rocket levels, the fuel resources would disappear after I collected more than the maximum fuel in the resource-collecting level. It wouldn't say "MAXED" either, it would simply say the number of fuel I have. After leaving the resource-collecting level, it would say "MAXED", but it would still be above the maximum. This has the effect of disabling the access to the launch pad. When the bug is active, re-entering and leaving the resource-collecting level sets the fuel to its real maximum and the launch pad can be accessed normally.
Also, a minor oversight: during the copper rocket level, when the resource-collecting level is the wasteland, there are two pieces of charcoal sitting in the same spot on the platform with the first badger. One floats a couple of pixels above the one in front of it.
All in all, another very fun game.
EDIT: Just discovered the bug is caused by the fuel tank level, not the rocket stage. Not sure when the problems start, but I can tell you up to level 10 fuel tank, the resources are maxing out correctly.
EDIT 2: Am at fuel tank level 20. Things maxing out correctly again. The problem seems to lie somewhere between fuel tank levels 10 and 20.
EDIT 3: It happens with fuel tank levels 15 and 16. 17+ seems fine. It appears to me that the amount of fuel the resources give you multiplied by the amount of resources needed for a full tank go over 100% fuel tank capacity.
EDIT AGAIN (SORRY): Everything seems to be working fine in my current playthrough, but there was definitely something going on with collecting more fuel than maximum. Now I don't know what's causing it.
Spoke too soon. Happened again. This time I can tell you info because I paid attention: copper rocket/wasteland level, fuel tank 20, engine 15.
Another update: This bug is behaving strangely, so I can't pin down what's causing it. This time: Twig Rocket, Fuel Tank 5, Engine 5. Grass resources disappeared untimely, this time before I reached maximum grass. Never happened before.