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.
Profitability is constantly changing. It is nothing abnormal if your profitability changes on a day-to-day basis or even on an hourly basis. The profitability change is mostly a response to the change of values of cryptocurrencies on global markets.