Linux – pvresize doesn’t seem to resize after increasing the size of the underlying block device

I increased the size of the partition which I'm using as a LVM PV, but running pvresize doesn't seem to see the extra space:

cuttle:~# fdisk -l /dev/vda

Disk /dev/vda: 21.5 GB, 21474836480 bytes
255 heads, 63 sectors/track, 2610 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0x00027dbb

   Device Boot      Start         End      Blocks   Id  System
/dev/vda1   *           1          31      248976   83  Linux
/dev/vda2              32        2610    20715817+  8e  Linux LVM

Which says that vda2 is about 20gigs

cuttle:~# pvdisplay
  --- Physical volume ---
  PV Name               /dev/vda2
  VG Name               debian
  PV Size               4.76 GiB / not usable 3.08 MiB
  Allocatable           yes (but full)
  PE Size               4.00 MiB
  Total PE              1217
  Free PE               0
  Allocated PE          1217
  PV UUID               tehZic-5vfN-rsrm-B8lN-lpgc-yQT1-ioH1V0

So currently the pv is about 4-5 gigs

cuttle:~# pvresize -v /dev/vda2
    Using physical volume(s) on command line
    Archiving volume group "debian" metadata (seqno 12).
    No change to size of physical volume /dev/vda2.
    Resizing volume "/dev/vda2" to 9975981 sectors.
    Updating physical volume "/dev/vda2"
    Creating volume group backup "/etc/lvm/backup/debian" (seqno 13).
  Physical volume "/dev/vda2" changed
  1 physical volume(s) resized / 0 physical volume(s) not resized

pvresize says "No change to size of physical volume /dev/vda2"

cuttle:~# pvdisplay
  --- Physical volume ---
  PV Name               /dev/vda2
  VG Name               debian
  PV Size               4.76 GiB / not usable 3.08 MiB
  Allocatable           yes (but full)
  PE Size               4.00 MiB
  Total PE              1217
  Free PE               0
  Allocated PE          1217
  PV UUID               tehZic-5vfN-rsrm-B8lN-lpgc-yQT1-ioH1V0

and the size of the pv hasn't changed.

Not sure what else I might do to use the extra space. I suppose I could resize the partition to the size of the pv, then add a second partition, but it really seems to me that what I'm trying to do here should work.