If you can remove the loose tiles without destroying them, they can be reset and regrouted. Use a grout removal tool to detach the loose tiles; then clean, reset and regrout.

Thanks worthy. Reset using what exactly? Do I need to dig out the old mortar behind them? And if not, won't it make the tiles stick out further?

Yes, carefully remove the old cement and use a bit of mastic to set them again, being careful to keep the same spacing for the grout lines using a bit of cardboard or plastic spacers.