There are a lot of things that can affect the bond price. For example, the change of the credit score of the bond issuer can have a direct impact on the price. If for example, you bought a bond yesterday for $100 and the credit score was triple A and you try to sell it today but the credit score has fallen to A, the offers are going to be lower than the originally paid.
In addition, a bond that gives a string of payment to the bond holder will have its priced decreased as more coupons are paid. Finally, the interest rates (connected to the credit score) can cause the bond price to move in the opposite direction. If I can buy a bond today that offers interest payments at 10% (quite high but just for the sake of this example), why would I want to buy yours that gives 5% (both bonds are issued by the same government, company etc)?