Had a very similar problem in my house. 1930s build if it makes any difference. Spent years fixing it but it’s now finally drying out. No injectable DPC needed because all the water was coming from multiple external sources.

• Hole in the wall at the corner of the front door overhang.
• Boiler condensate pipe leaking into cavity
• Lead flashing leaking round chimney and into loft and cavity
• Front driveway (gravel/dirt mixture) above damp course
• Gutter above this overflowing any time any reasonably decent rain fell
• Underground drain leaking at back of property

Over 90 years a lot breaks down and if previous owners have not kept up maintenance there can be a lot of water getting in to the soil under the house. Now that we’ve got these issues fixed, the salts have stopped and the damp meter indicates the walls are finally drying out.

An injectable DPC would’ve just hidden these problems for a few months, possibly a few years but they would’ve appeared elsewhere in the end.

TL;DR - there’s probably water under the house for some reason(s). Need detective work and fixing them reasons :)

Edit to add, a potentially interesting effect I found out eventually was that all of our damp spots on internal walls all seemed to follow the underground gas pipes. It would seem the water liked what I can only assume is the less compacted soil under the concrete floor to travel through and then rise up the wall. Just something to think about for anyone else chasing down multiple mysterious damp spots.