Hi Yassine,
it might be that there are invalid values in this timeframe. You can try to change your formula to $as32.validValues(). This will remove invalid values from the signal and interpolation should work again. If the gap is greater than the max interpolation setting you can use $as32.validValues().setMaxInterpolation(4weeks) which first removes invalid values and then changes the max interpolation of the signal.
Regards,
Thorsten