Inconsistent mouse-over behaviour

I’m using the latest version of the plugin. I recently uploaded two batches of about 100 new events, in two csv files. Previously, all our older events showed a box with more details when you put the mouse over the event in calendar view. With the new batch of events, the behaviour is erratic: many do not show the mouse-over on our current WordPress 2014 theme http://www.rfyc.org/events/2016-06/ , and most of them don’t show mouseover on the mockup of our new theme
http://rfyc-staging.re-hosted.co.uk/events/2016-05/ .

Analysing them more carefully, all of one batch I imported display OK on both themes, and a variable number of the other batch displays depending on theme. Looking at the csv files there is no obvious systematic difference that could account for the different behaviour between the two batches.

Any ideas? Anyone else got this problem?

https://wordpress.org/plugins/the-events-calendar/