Not all plugins can be found after we switched to KoServiceLocator.

In KoServiceLocator::init() KService::name() is used as an unique key
while finding *.desktop files. I.e. the Name field is used.
But as the script [1] can show you (run it in calligra/), there are
numerous services in calligra (not just Kexi) with the same Name
field. [2]
It's nothing unusual, services typically differ in Comment more than
Name. And kde-workspace has many duplications too.

Solution: use both library name and name as a key.
(yes, there are services sharing the same library name)

Reviewed for 2.8 in this thread, it's not different in master:
http://mail.kde.org/pipermail/calligra-devel/2014-September/011865.html