Purity: Doubt About Water Reaching Under an Obstruction

Practical Laws of Islam as per the teachings of Ayatullah Sistani

Ruling 295:

If on some parts of the body on which wuḍūʾ is performed there is an obstruction under which water sometimes manages to go by itself and at other times does not, and if one doubts after performing wuḍūʾ whether water went under it or not, then in the event that he knows that while he was performing wuḍūʾ he was not aware of water going under it, the recommended precaution is that he should perform wuḍūʾ again.

-Ayatullah Sistani, Practical Laws of Islam, Purification (Taharah)