Assalaamu alaykum. Sometimes, when I wipe the back of my ears with my thumbs in wudoo (ablution), my thumb does not cover all of the back of my earlobes. Is my wudoo still valid, or do I have to make sure that I wipe every small part of the back of my ear when I do wudoo.

All perfect praise be to Allaah, The Lord of the worlds. I testify that there is none worthy of worship except Allaah and that Muhammad, sallallaahu ʻalayhi wa sallam, is His slave and Messenger. 

According to the majority of the scholars, wiping the ears in wudhoo is an act of the Sunnah. If the person performing wudhoo did not wipe his ears at all, then his wudhoo is still valid. However, it is recommended to wipe the entire front and back of the ears, wiping the back with the thumbs and the front with the index fingers. This is the perfect manner of wiping the ears in wudhoo as was reported from the Prophet, sallallaahu ʻalayhi wa sallam. However, your wudhoo is valid in all cases because wiping the ears is an act of Sunnah, as mentioned above.

Allaah knows best.