Porcelain paving sounds hollow

At the start of Dec 2018 we had our garden landscaped and around 60m2 of vitrified porcelain pacing.

Around 10 Paviors on one half of the garden sound hollow with a couple of the show tiny amounts of movement.

The landscaper is coming to have a look next week but I wanted to see people’s opinion... anyone else ever had this with porcelain?

I'd think it's come away from the bedding..
I used pva in the mix when I laid slate paving in a wet mortar

Hopefully it's just a bit of poor bonding so a re-bed should sort them.