Parking sensor wows

Had a long bleep from the rear parking sensors which stopped so had a listen & could not hear any clicks from the middle two. Took car back to garage where I brought it, 100 round trip, they fitted two new centre ones & it worked until I got home but now it just bleeps all the time as if something is behind the car.

I'm starting to regret buying this car even the simple jobs are hindered by daft design like having to take bumper off to get at sensors, manifold to change plugs & very tight access to change bulbs.

Before I waist my money driving the car another 100 mile to get a fix do any of you have any suggestions as to what I can try to get the parking sensors working ?

Thanks