I hunted two different elusive people for ages, when I finally cornered them:
* Had gone under a different surname, mum and child, for 2 censuses, apparently Mum "married" another ..... took ages to track that one down, had birth record, and marriage data, but in between?

Zilch!
* Mum and son hitched up with another man, ( apparently mislaid real Dad) and ALL THREE , Mum, son and Mum's " fancy man" (I love that term!) adopted that rare surname "Smith" as an alias from shortly after son't birth until Man died, and was buried under his real name .... and younger bro/half bro of son resumed real family name, which was fortunate, for original son also reclaimed his real family name! That took 6 years to track out.
-So don't give up! The chances are they'll be out there somewhere - and if anyone can fish them all out from under that radar, someone on here WILL manage it!