Do all mandevillas have runners on them
We have ours in two big pots. One has tendrils on it and the other does not.
All healthy mandevillas develop new shoots and tendrils. Does the plant in question look healthy? A plant with compromised health may not grow as well.
Your one plant is in active stage of growth, the other one is probably just being a late bloomer. The best thing I can prescribe is patience and ensuring that your mandevillas receive the care outlined in the following article: