Sounds like you are doing the prepress and layout for your jobs, then you send those files to your plate making site.
In order to make plates something has to be rasterizing and screening the files you send. Since you are sending Normalized PDF and PDFPLA files (Esko format) my sense is you have an EskoArtwork RIP on the other end. The question is then what "flavor" RIP are you using on that end?
Give Esko support a call. They'll be able to lookup your configuration and let you know if you have what it takes presently to make the TIFF's, or what it would take to do so.