I can't sleep because monsters are nearby, they are hidden somewhere around my house in some walls. It happened before but it stopped after a while but now it came back. I know they are in some hollow spots sometimes but I can't find them.
In recent and current versions of Minecraft, you cannot sleep if a hostile mob is within 8 blocks of the bed horizontally (along each axis) and 5 blocks vertically. If a monster is nearby, even through a wall, the message "You may not rest now, there are monsters nearby" will be displayed and the player will not be able to sleep until the monsters go away or are killed. - http://www.minecraftwiki.net/wiki/Bed
The easiest way to solve this issue is to fill in the hollow spots with dirt or another material to ensure the monsters won't spawn, as well as lighting the area around your room or home with torches.
You should be able to find the monsters through the sounds they make (unless it's a creeper), so make sure you're listening out.
You shouldn't need to patrol too far from the bed, and ideally go back to the places around the walls that you've probably hollowed-out as it's likely they're in there. Listen out for them, smash open the walls and open the cavities (watch out there could be loads in there, depending how big the area is). Then fill it up with dirt, or leave it open to the light so that they won't respawn in there.
So, find them by sound, open up the areas they're in to the elements, kill them off and go to bed!
When you can't go to sleep on Minecraft Pocket Edition you must be trying to sleep during the day or there are monsters nearby. So wait until it's nighttime to go to sleep and kill all the monsters around you to go to sleep but watch out because you can't hear creepers when they are around you and they might blow up your house if you are to close to them so be careful!