Patio break up - Flags keep loosening

[novice northerner]

Hope someone has a solution to our problem
We had a patio laid 1 year ago consisting of polished indian sandstone on a prepared concrete slab. The builder has used a bed of mortar on this slab and then mortar jointing.
when we tapped it certain areas sounded hollow and before long some of the flags loosened. The mortar was scraped off and relaid but the same is happening again. The flags were initially laid in cold weather. ?Is it that the cement is not adhering to the flag , ?is it that water seeping underneath the flag has no where to drain and then loosening the flag or is it poor application in the wrong weather??
Thanks for any reply

[joydivision]

is there a full screed under the flags, or have they been layed on 'spots'?

Allan

[Dave_L]

The hollow sounding slabs sets the old alarm bells ringing.............

[seanandruby]

You say: they were laid a year ago. When did the flags become loose? It is important to know because a lot of patios were ruined by the rigours of the winter weather.

[novice northerner]

thanks for the replies,
the patio was laid on what looks like a thin bed of mortar. The patio level is to match up with the tiles in the lounge which is accessed by folding sliding doors so having laid the slab of concrete there is not a lot of space for manoevere if the levels are going to match so probably this is the reason for the thin bed of mortar on top of the slab
The tiles first started to loosen in the summer though it was a pretty wet summer. These were replaced and now started to loosen again this spring. Could it be due to poor adhesion to the mortar?

[Tony McC]

How thin is thin? Less than 12mm?

[novice northerner]

The mortar bed is 10-15mm thick Tony I went around again today tapping all the slabs with a broom handle and most sound dull and solid and others particularly the bigger slabs sound dull in parts and hollow in others.

Cheers