|I suspect you also need to shrink the extended partition. You have probably only shrunk the logical partition.|
Thus the space you have freed up is not unallocated space. It is free space within the extended partition and therefore only available for logical partitions not primary partitions.
Also which end have you shrunk it from, the tail end or the head end?
To add unallocated space to a partition requires the free unallocated space to be contiguous with the partition that is be resized.
☺ When everything else fails, read the instructions.