Acrobat watermarking doesn't work

My watermark function on Adobe Acrobat doesn't work properly - I can add a watermark but it doesn't save. I add them through Edit PDF - Watermark - Add. Have tried this on both PC (Lenovo thinkbook) and Mac (iMac), both of which use the same CC account, so maybe it's to do with my account? I'd be grateful for any suggestions. I've resorted to asking team mates to watermark PDFs for me or adding watermarks in InDesign, but that's a faff.