UK mail - not called for and address squiggled through??

[joeym90]

I am in the UK and last month sent quite a few requests, 3-4 have come back with a sticker saying not called for and my address had been squiggled through, 3 of the 4 the stamps weren't stamped through and 1 was. Before opening I thought it meant they had bee returned unsigned because the person couldn't be reached but all of them had been signed and sent back so I'm a bit confused, I had to pick them up from the post office but don't understand why the stamps hadn't been marked and why they hadn't been post through my letterbox like they usually are. Anyone know any explanations behind this or had similar experiences?