We had new windows fitted in June 2019 and have since had to have a small side window repaired due to it cracking.
It has happened for a second time now and we are worrying that its a bigger problem, maybe with the house itself.
Has anyone any experience with windows that keep cracking or know what could be the reason for it? 
Best Answer
OK - Either this is a huge optical illusion or my eyes tell me that the window looks like it is being squeezed at the bottom.
It is simple - windows should never crack like this unless something hits it or it goes through a major earthquake. Let's remove the issues with your house. It looks like there is movement and it looks like this may be seasonal/temp related. This is completely out of scope for the window issue. You may have a huge foundation issue, maybe it is no big deal - doesn't matter.
You need a new window. It needs to be sized correctly. There should be space on at least one of the sides and either top or bottom. Windows are not packed into an opening. They are put into an opening and screwed into place using shims. This is to give the window the ability to move and to get a more plumb install.
The other option that you have it is use the same window but shave off room on the outside - I do not recommend this though because it could decrease the integrity of the unit which seems to already have issues. But if you keep putting glass panes in there they will certainly keep breaking.