There are two reasons why this might happen. First, if the filling is not thick enough before you transfer it to the pan. Make sure to not mix the mascarpone for too long before adding the other ingredients. Then, after adding the lemon juice, mix until the filling has thickened and has the consistency of Greek yogurt. It shouldn't be runny. The second reason why the filling is not setting is that the pie hasn't been in the fridge for long enough. I recommend chilling it overnight.