I've only had this problem recently...and only on certain Hoyas:
fischeriana (big time)
macgillivrayi (big time)
pubicalyx 'Red Buttons'
Most of them are putting out new growth, some one really long vines (fischeriana and macgillivrayi in particular). I was excited about this until the newest leaves started falling off without any sort of trauma. It's frustrating! They're not over or underwatered, they're getting enough lights (the same amount of light they've alway gotten). I don't know what I'm doing wrong. Help?