Who's the manuf ? You mentioned that when installing the plank you have to tilt the adjoining plank up to interlock. This somewhat tells me the type of profile of the t&g. Although some profiles that you are suppose to tilt, you can also lock while flat with a tapping block. That being said ...Depending on the core layer and the t&g profile, using a tapping block, you run the risk of breaking the locking system. Which wont help you anyway, cuz the whole floors installed
It's also possible that chunks of wood from the locking system are busted off in the t&g right now, you obviously can't see.
Here's how to fix it...
1) Lift out the planks that are not locked
2) There is a lip or beed of wood on the groove profile of the plank, ( this is the lip that makes you tilt the adjoining plank, in order to interlock). Use a scraper or sharp chisel and remove the beed flush with the bottom of the groove profile.
3) Take some t&g adhesive, ( I recommend Titebond ) . Run a generous beed of glue in the groove, set the plank back in, and weight it down.
If you have any scraps...You'll see what I'm talking about before pulling the planks.