Over wintering a Dipladenia Plant
I brought in a dipladenia last winter. I put it outside in the summer. It had beautiful green foliage, but not even one blossom. I've brought it in again for the winter. Is there any way to get this plant to flower again? I bought another plant this spring and that one did flower. I have brought that one in also. How do I get this one to blossom next year?
You did not say if the plant has ever bloomed. If it bloomed last winter when you purchased it (or it had blooms on it when you purchased it), it is possible it needed to recharge. Hence the foliage with no blooms.
Bring it indoors this winter and hopefully you will have better luck next summer. Don't expect a bloom in the winter - there is not enough light.
When you put it outdoors, make sure it gets plenty of sun. Read here about caring for and wintering dipladenia: