Answer: It sounds like you are doing things right. Could it be that your allamanda is one of the new dwarf types? I have grown these and they get about 18-24 inches tall. Take a close look at the distance between the leaves on a stem or shoot. The standard allamanda will have leaves over 1" apart while the dwarf packs its leaves in much closer (maybe 1/2 inch apart).
If you feel the plant is a standard type, I'd give it some time to decide if it wants to take off and grow. By the third year these plants can really begin to get lanky, so maybe yours will get the idea soon!
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