This may or may not apply to your specific situation, but it is generally worth knowing:
(1) If the PDF has a color profile (*.icc) embedded, then many printers will produce CMYK "rich black" instead of pure K black. That is very undesirable for text, as it is intended for images. It will resemble rasterization.
Most TeX-produced PDFs will not have an *.icc embedded. But it is possible, especially if you are using PDF/X, and do not do it in a particular manner.
Do you have access to a program such as Adobe Acrobat Pro? Not the free version. If so, open one of your problem PDF files in it, and see if there is an Output Intent with embedded color profile. If that is the case, strip out the profile, and like magic your print will improve.
(2) If a page has both raster image and vector (text) objects, then any vector object that overlies any part of a raster image may be rasterized. In some cases, merely approaching the image will cause rasterization. Of course, everything is rasterized during print; but I refer to a lower-resolution rasterization that precedes final print rasterization.
This applies even if the underlying image is white or transparent there. In particular, it may apply if you are using a background or raster watermark.
(3) The above situations are not specific to TeX. If your printer has seen problems more frequently with TeX files, I surmise that the reason has to do with the content of the files, rather than the program used.
Also, some TeX users may still (in this day and age!) be using bitmap fonts here and there. The presence of a bitmap (raster) character may spread rasterization around to nearby vector text, like an epidemic.
(4) The information I provided above does not come from my own deep knowledge of the technology. Instead, I have noticed rasterization issues raised in other forums, by book writers (non-technical). They never heard of TeX. My information is a summary of responses from those who know.