In this paper, we study the effects of temperature and electron effective mass within the barrier on the bias dependence and sign-change behaviour of the tunnelling magnetoresistance (TMR) in ferromagnetic junctions. A significant decrease of the tunnelling magnetoresistance with increasing temperature is obtained, in accordance with the experiments. In addition to the height of barrier potential (φ) discussed in our previous papers, the electron effective mass (mB) within the barrier region is found to be another important factor that physically controls the sign-change behaviour of the TMR. The critical voltage (Vc) at which TMR changes sign will increase with φ and decrease with mB. Furthermore, both the zero-bias TMR and Vc will decrease if the temperature rises. These results would be of practical use for experimental investigations.