Hollow patches in freshly plastered wall

[Ian79]

Some advice from the experts please:

 

We have just had a single story extension renovated, including complete re-plaster.

 

 On painting one of the , I noticed several (maybe 5 out 6) patches that sound hollow on tapping - typically 5cm across. They are at different heights in wall. The worst one actually collapsed when I (relatively gently!) prodded it - see photo. Apart from that one, they are not obvious now painted, except under certain light, but I’m just trying to understand what the problem is. Is this an early sign of damp (walls were plastered at least 6-7 weeks ago and are completely dry) or is there some bad workmanship that could explain this? Keen to understand ASAP before our builders finally depart this week - wanted to seek views on here before approaching them, as they will no doubt be dismissive!
 

The wall is a cavity wall (1970s extension)  but is also a retaining wall as it is below neighbours ground level which is half way up it. There were signs of some mould here previously behind furniture, but we put that down to simply being an unheated / unventilated room over the course of last winter.

 

 Any thoughts / advice welcome - especially whether we should ask the builders to investigate…

 

 

[joe90]

I had a few of these, called the guy back and quickly rectified, could find no cause but the guy said it sometimes happens. @nod  will know better 👍

[nod]

I agree with Joe Very easily sorted out 

Probably take him half an hour on his way home