We would need much more information than you provide. Solid lines? Thickness (as measured in Acrobat's Output Preview=>Object Inspector or even from within InDesign? Line color? Printer type and resolution?
Can we assume you are using Acrobat or Reader to print?
Both solid and dashed lines, though mostly occurs with solid lines. Thickness appears to be as little as .1 or as much as .3, though lineweights are from the Illustrator linked file and some images are scaled to fit page of indesign file. Line colors are a variety: black, gray, or 50% black. There are several files with different combinations of the above. The printer type is an HP Officeject Pro 8600 and resolution is believed to be printing at 600 x 600 dpi from Indesign or using Adobe PDF Preset [Smallest File Size] settings when exported to PDF and viewed/printed with Adobe Reader
The lines are probably just too fine for your printer to reproduce. For example, a .1-point line is just 1/720th of an inch thick - that's smaller than the 1/600th inch dot that your printer can print. If an imported file with a fine line was reduced in InDesign, the line might be even thinner than that. Acrobat Pro can identify (see Post #1 above) and even fix fine lines in the PDF, but I don't think Reader can - I don't have a recent version of Reader with which to test.