Fresh install of lubuntu synaptic is missing the search box in the tool bar

sudo apt-get install apt-xapian-index
sudo update-apt-xapian-index -vf

From: http://ubuntuforums.org/showthread.php?t=1178974

If the other answers don't help then try:

sudo apt-get install --reinstall synaptic
sudo dpkg-reconfigure synaptic

It worked for me.

forums linuxmint suggests installing apt-xapian-index when the box is present but grayed out. This solution also works in this case where the Quick Filter box is completely missing.