Gaps in grout

[SheWoreARaspberryBeret123]

Can you just plonk more grout on top of old floor tiles to full in gaps?

Or do you need to remove it all & start again?

Depends. If it is just a couple of bits missing I would just do those bits. If it is a bit rubbish over the whole floor I would take it all out and start again. The holes need to be big enough for the grout to get into properly so you might need to enlarge them first.