They need heat and humidity to grow, so just wait - that's all you need to do.

UPDATE: I winter-sowed my seeds on Feb. 2 and they sprouted their first two leaves yesterday.

Update on mine,STILL only 2 leaves on each,but healthy looking! I hope they start to grow soon,I am looking forward to having these in my garden this year.

I've found that KELP can help growth of seedlings. Start with half strength of manufacturer recommended dose to check response of plant then gradually increase to standard dose.
Incidentally, my I. andersonii is 3 years old and hasn't flowered yet. I'm hoping to see some blooms this season.

I can see why there is interest in this species. The flowers are attractive and it makes a fat root. My other fat root / caudiciform morning glories took a couple of years before they made flowers.