You definitely have to be doing something wrong, as /cast Spellname has worked with Healbot as long as I can remember (years after years), never had any problems with it.
But just to make sure, try to follow these steps:
Create a macro named "HBRejuv"
To that macro, paste the contents:
Open Healbot spells tab and add "HBRejuv" to the field that matches the buttons you wanna use it with (I personally use mouse right-click without modifiers)
Works 100%. If not, might be good idea to check auto self cast settings and so on, but those shouldn't mess it up. Maybe some other addon could also be intefering with that?
When you get it working, you might also wanna add more to the macro to best fit your needs, for example something like
Also remember @hbtarget targets the character whose health bar you hover over in Healbot UI, it doesn't work for mouseover targeting characters in-game (except those Healbot bars).