Bricks sinking - Driveway sinking after 4 months

[Sover]

We had a brick drive installed only 4 months ago by a registered Marshalls installer. A few weeks ago a single brick sank about 1 cm. Some days later it had fixed itself and risen. Now a whole section of about a dozen bricks have sunk and some of the bricks on the borders rise and fall when walked on. He has promised to return to fix the problem but in advance we'd like some idea of what is causing it and what we should expect him to do.
Thanks in advance for any ideas

Pic of the problems can be seen here.

My Drive

[carlbeardsmore]

Did the contractor or yourself top up the sand in the joints after a few weeks of the drive being laid?

There does not look like there is much sand in the joints.

It is not that hard to lift a few square meters and relay them.

Did you see how much sub base went down before the screed sand?

[Sover]

Hi Carl, thanks for the response. The sand has never been topped up and it was not mentioned to us that this may be necessary. At least now I know what to expect them to do when they finally get back to us. Strange thing is though when we got up this morning everything is fine again and level! They seemed to put down a good base level of sand when they did it.(Couple of inches maybe, which we thought looked OK) Thanks again for the advice and will see how they progress.

[bobhughes]

A couple of inches of sand.... OK. But what is under that?

Do you have any water drainage problems or tree roots?

[carlbeardsmore]

The jointing sand will settle over a few months. You then have to top it up. There is no need to use a vibrating plate again. The joint sand really adds strength to the pavement and without topping it up , some blocks can start to rock.

[lutonlagerlout]

that is really odd,i have seen driveways dip but never as regimented as that,could you perhaps have a leaking pipe under the drive??
normally once they sink they stay sunk!!
its curious thats for sure
regards tony