Second ability in macro sometimes not working

[Deleted]

#showtooltip Haunt
/cast Demon Soul
/cast Haunt

Demon Soul isn't on the GCD as far as I know, but Haunts gets cast, but Demon Soul not every time (when it's off cooldown). My pet (required for Demon Soul is out).

How can I improve the macro?

[Judge40]

Try:

#showtooltip Haunt
/cast [@pet,nodead] Demon Soul
/cast Haunt

Not sure it will fix the problem you have, but its a little more friendly when you don't have a pet out.