PHP5 packages can be installed on one 16.04 system but not on another

[Chris Green]

I have two xubuntu 16.04 systems both with LAMP server software on
them.  On one I can see and successfully install a number of PHP5
packages, in particular php5-sqlite, on the other it can't see this
package at all using apt-search or synaptic.

I have checked through all the repsitories in /etc/apt and, as far as
I can see, they're identical.

What is wrong?  I need php5-sqlite on both systems.

-- 
Chris Green