So lets say in a game of mine i had the party separated into two groups half elf (rogue) Warlock (Fairy) and Dwarf (paladin) Orc (barbarian) and i had the first group with the rogue and the warlock get blocked in by a massive boulder. So they were basically stuck in a cave between some pirates and a boulder. They did not want to fight the pirates yet so the fairy squeezed past the boulder and then the rogue tried to do the same but could not. So the fairy ofc rolled a nat 20 and a 18 on an athletics test with disadvantage. What are the odds.... so the fairy pushed the boulder out of the way and let the rogue escape back to the group. How can this be justified? Do i just tell them that "there is no way for you two to push the boulder given the circumstances" is that just a flaw? What about if a barbarian rolled a 4 on an athletics test to break down a wooden door (dc 7) does the barbarian with an 18 strength bonus just fail? How can i make this system better? Do any of you have any homebrew rules or justifications for this?

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TLDR: Fairy pushes giant boulder out of the way. HELP!! Barbarian cant break puny door. HELP!!