The problem usually happens because of a resizing of the image or because the printer decides that it doesn't have the resolution to draw a one pixel line (which is how wide the missing staff lines are).
A couple of things to try is:
1) Select a different quality in the Paper/Quality tab of the print dialog
2) Try setting the print to Black and White in the Paper/Quality tab of the print dialog
3) Try downloading the file and using Adobe Reader to print
4) Try loading the pdf file into Word and print from there
If none of these work try downloading our viewer and the RMS file that is used for transposition or digital playback.
You can see a full online manual of the viewer product <a href='https://support.sheetmusicnow.com/hc/en-us/sections/360001890651-Sheet-Music-Now-Viewer-Manual'>here</a>.
You can download the viewer product from <a href='https://www.sheetmusicnow.com/pages/download'>here</a>.