If you are referring to the balance expressed in fiat currency (USD, EUR, RUB, ...), the reason for the decreased value lies in the value Bitcoin has on the global market. NiceHash pays you in bitcoins, but the value of the earned bitcoins is expressed over the website and in our software in fiat currency as well. This value changes whenever the value of bitcoin changes on the market. So, if one day, you had $10 of earned bitcoins and only $8 the next day, the reason lies in the lower value of bitcoin on the market and changed exchange rate.
If you are referring to the balance expressed in bitcoins, check if you have been recently paid. If not, contact us through support ticket system and we will answer you as soon as we can.
Please be aware that profitability is constantly changing. If you go to your miner statistics page and scroll down to Interactive history graph (see image below), you can see your profitability over time. Profitability fluctuates all the time because of the buyer's orders on the NiceHash marketplace. If buyers demand more hashing power, the profitability will go up. If there are fewer (or smaller) orders on the marketplace the profitability goes down. You, as a seller (miner) of hashing power, fulfil the buyer's order for hashing power - it's a very dynamic supply and demand relationship. Also, we recommend to focus on average profitability over longer periods of time, and not too much on current profitability. Keep this in mind when looking at profitability stats.