I would guess that the "cover" cardstock is too heavy for the printer.
Specifications> Paper Handling > Paper tray capacities
Although "cards" are listed as supported, there is a difference between Index card and paper manufactured as "Cover" stock.
Cover stock is heavier than Index stock.
There are articles on the 'net to explain how paper is manufactured, including the different types (Index, Text, Cover, etc). Different stock, types are measured according to the density of that stock.
One example of many available:
Gsm / paper weight (includes media type “card stock”
Calculate & Convert Paper Weights on Your Own Website
Reference and Resources – Places to find help and learn how to use your Printer
Clarification of terms and "what the printer can do" (Specifications)
Printer Homepage – “Things that are your printer”
NOTE: Content depends on device type and Operating System
➡️ HP Drivers / Software / Firmware Updates, How-to Videos, Bulletins/Notices, Lots of How-to Documents, Troubleshooting, Access to the Print and Scan Doctor (Windows), User Guides, Product Information (Specifications), more
When the website support page opens, Select (as available) a Category> Topic > Subtopic
HP OfficeJet Pro 8710 All-in-One Printer series
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