When you build a dungeon, you put rooms in it. You put treasures, clues, secrets, and other neat stuff. Sometimes, you make doors stuck, or locked, or otherwise difficult to open. Sometimes doors are secret or concealed.
When you do this, you create the possibility that the rooms beyond will not be explored.
Depending on your vintage and style of game, the players will often have a way to bypass your difficult doors: perhaps you play a version of the game that has a thief who can pick a lock with a lucky die roll. Perhaps the dwarf or elf or someone along in the band has a greater than average chance of detecting your hidden door or your stone block trap.
The PCs finding the hidden/difficult to open rooms hinges on several factors. Some of them involve the players thinking to look for doors or managing to find their way into this part of the dungeon. Then, it comes down to luck. Will the thief make his 15% roll? Will someone, anyone, roll a 1 on their 1-in-6 chance to find the hidden lever that opens the door? There is a possibility that the dice will simply not be in their favor.
In the case of stuck or locked doors, PCs can smash the doors down. This might create unforeseen consequences, like monsters investigating the noise, plus delayed time, which means those spells and torches and such begin to fizzle out...
If your hidden doors require the solving of a riddle or puzzle, player skill becomes more important.
What about hidden doors? There is a big chance they simply will not find them. If you require hidden doors to be found through narrative, then players not finding your hidden rooms is on them.
If you allow secret door checks, the dice may dictate that those rooms you created will never be found.
...of course, an unlucky random encounter and a cascade of bad die rolls always carried with it the possibility that your party will never see any other part of the dungeon...or, for that matter, your particular campaign.
Are you okay with this?
Do you recycle the dungeon levels or the missed rooms? Doesn't that invalidate their value as a reward? Doesn't that start us down the path to 4E's Treasure Parcels?
As folks around here are fond of saying, allowing for failure is the only way to give success meaning. Allowing for some stuff to simply never be found is the only way to have action/inaction/success/failure carry any weight.
Today, while I was cleaning out my basement, I threw away the map to the Keep of the Stone-Scream Clan, the mostly unexplored dungeon in which my Pathfinder group suffered a TPK back in June. I spent hours on that dungeon, making level after level, and the party died on the third level. If I ever give Pathfinder another go, and I decide to use the same campaign world, I have decided that the deaths of the adventuring PCs meant that the last Stone-Scream sorcerer was able to bring his demonic master into the mortal plane, and that the mountains are now an uninhabitable demi-plane of madness.