I have not grown this Eriostemma hybrid so I have no first hand experience. Some Hoyas just take a long time to start growing.
Does the cutting still look healthy? Most growers add a Tsp of dolomitic Lime to the potting mix (4-6" pot) for Eriostemmas. If the pH of the potting mix is not quite right for the plant it may be causing problems, that's just a stab in the dark.

How often are you watering and how much light are the plants getting?

Mike

Sorry, I don't grow this one, but reading up on it, no one mentions it being a particularly troublesome Eriostemma. Which makes me wonder whether it's not Monette per say, but your start of it.

Sometimes I start a cutting or receive a plant and it just inexplicably takes a super long time to really take off. If it were mine, I might consider taking a small cutting off the top and starting it next to the first. It's a bit drastic to restart the whole plant, but if your new baby took off, you'd know the main plant was just one of those fussy ones and you could give it its time. Maybe the hormones from the new rooted cutting would even stoke its fires.

I really disliked nicholsoniae for the longest time because my cutting did this, and then one day, maybe after a year and a half or so, it just started growing and now it has practically caught up with its peers.

Well, anyway, that's just a thought. Don't take it as law or anything. :P Good luck!