Working with gelantine can be tricky, so if you have never tried it before then make only the mousse and use it for a delicious dessert with some fresh berries. This will help you troubleshooting any problems you may run into and you wont end up with a spoiled mousse that should have been used in a cake.
It is also important that the berry mixture is warm but not hot before the gelantine sheets is added. It will not give you any problems melting the gelantine, but a too hot berry/cream mixture, will give you a very liquid mousse that is not pipeable for cake filling.
Cakes filled with raspberry mousse must be kept in the refrigerator. Take the cake out 1 hour before serving.