Bond prices can go up or down based on factors which may include financial performance by companies, interest rates, changes in the economy and special events e.g. Brexit or a number of other variables.
WiseAlpha updates the prices of the bonds each business day between 8am and 10am that correspond to each Fractional Bond. It is important to bear in mind that while prices might fluctuate during its life, bonds are contractually required to repay at par (i.e. 100 or face value) so there is an expectancy of capital return which is not the case with equities. Therefore price fluctuations tend not to be as volatile.