Tokens (i.e. placeables) not recognised...

In one particular project I'm working on, "tokens" i.e. placeables such as dates, numbers and acronyms are not being recognised in the source text and therefore I can't use Ctrl+comma to insert them as I type. I have checked and double checked the TM settings and all the relevant boxes are ticked. In fact, using the same TM for another project, placeables come up marked in blue and everything is normal. What could I be missing? I am using Trados Studio 2021 SR1 - 16.1.4.4184.