I'm not able to mine as much Bitcoin as I was able to a couple of months ago I’ve been using two AntMiner S5 ~1155Gh/s and was able to mine about 0.02 Bitcoin.
 However, recently I’m just able to about 0.01 Bitcoin.

I use F2Pool for mining. I thought first the other one isn’t working but it does.
Does anyone know what is going on here?
As more people contribute to mining, Bitcoin automatically adjusts the "difficulty" so that the number of new bitcoins created by mining is roughly the same over time. The difficulty has increased rapidly in the last few months, and in fact it has nearly doubled since November 2015. This matches with your performance numbers. Your existing hardware will become less useful over time.