Hidson

Hi I was wondering if anyone can help or is having similar issues, for the last 10 days every time I start the car it says please clean front parking sensor and something is in the way, so they don’t work I’ve tried cleaning them but their nothing obvious on them, also not sure if I’m being stupid but I only seem to see two front sensors I thought their would be four, the rear ones work no problem any help or advice would be appreciated