Stunted wings

[Masterfincher]

Hi all,
Over the last few months I have had a Gouldian and silver-headed nun fledge with their left wings deformed - basically stunted - so they can't fly, (although they try; they are now very good at hopping!). Otherwise they are healthy and seem happy living on the aviary floor and climbing the grass and trees.
Any idea what might cause this? I have had plenty of others - same species and different - fledge with no issues.