Blocks/Animals don't always drop

Hello!

So at first I was on creative mode, building my house. Everything worked perfectly fine. Today, I switched to survival mode. I mined a tree but didn't pay attention to if I was getting all the wood I was supposed to get. A bit later with a stick that fell from the dissipating leaf blocks, I hit a fish and collected the salmon that was dropped. I hit a second fish with the stick again, but nothing dropped. I was a little confused but didn't question it, until I made a wooden pickaxe and began mining some stone. Sometimes the stone would drop, and sometimes it wouldn't. After mining a block, cobblestone would drop, but after mining two more blocks, nothing would drop; after mining another block, a piece of cobblestone would then drop. Same thing happened when I then tried mining some coal. Instead of collecting, let's say, 9 coal, I would collect 5 instead. One mined block would drop coal, another mined block would not.