The money which you need to folk out to purchase mining hardware is expensive. Because of new technology being introduced the difficulty has risen by a lot. It's just going to continue to get harder. You might buy hardware which will bring in around 1 BTC this week, but a month later it could bring in something like 0.2. You have to be very careful when going into mining. You have to take into count electricity bills, break even of hardware and must have the technical know how to maintain the equipment. Many have reported having their hash rate slow after time, that's normally because it was either not set up correctly or it is over heating. There are many things which need to be taken into consideration before proceeding.