Can't add lights in LumenRT?

Hi all,

what am i doing wrong with the lights?  i want to add a series of lights to the underside of this bridge but after having placed 1, clicking the '+' does not let me add any more, see the image (and gif link for better moving example) below.  Any advice would be gratefully received.

Gif link: https://imgur.com/a/ghshq5L

screenshot: