ProTools 10: problem with identify beat to create tempo map.

I am trying to create a tempo map for a song recorded without a click track. So far so good, but after the first 25 bars of successfully clicking "identify beat" when the song is on the down beat of each bar, I get the message "the bar | beat marker could not be created, because they are either too close or too far apart from a neighbouring beat marker to support a valid tempo".

I don't know why this is - though I do note that the bar number appears to have jumped upwards in a non-linear way: I attach a screen-cap.

I thank anyone who can help enormously in advance!

MacBook Pro (mid 2010) 15" 2.66GHz i7, 8GB RAM
ProTools 10 (version 11.1.3.325)

Attached Thumbnails