There is a limited amount of inventory slots in Minecraft PE. Have you ever finally found that coal you'd been searching for only to find you can't pick it up because all of your inventory slots are used by bricks, axes, stairs, etc? There is a way to create over a dozen more slots-- and it's easy to do:

Open Minecraft PE on your device.
Before playing in SURVIVAL mode open up a CREATIVE mode map.
Close the CREATIVE mode map.
Open the SURVIVAL mode map you wish to play: you will see over a dozen empty inventory slots.
You must do this every time you restart the game in SURVIVAL mode or you will lose the inventory in those slots. (NOTE: Today I accidentally hit the "home" button on my phone while playing Minecraft PE and it took me out of Minecraft and directly to my home screen. I simply tapped the Minecraft PE app on my home screen and I was back right where I left off, with all of my inventory in tact. This is because I did not quit the game, my phone merely switched tasks for a moment.)

I am in the habit of opening in CREATIVE mode before playing in SURVIVAL mode: this trick works every time., and all of my inventory is right where it belongs. Happy gaming!