This issue could occur because of different reasons. The main issue is that the antenna can not capture all the modulations. An ISO14443-A reader drops the field the full 100%. The tag instead, only uses a subcarrier and adjust the field not more then 10%. This major difference makes it often easier for the antenna to catch the reader signal, but not the tag.
First of all, you should check if your HF antenna returns a acceptable value using the "tune" command. More information about this command is available in the antenna section of the manual.
Secondly you should start playing with the positioning of your reader, Proxmark-antenna and tag.
From experience I should vote for a setup where
- The reader is on the table.
- The Proxmark-antenna directly on top of that.
- Add 1cm space, by placing a small book / some bubble plastic / empty jewel case.
- Put the tag on top of the space-making object.
This often gives the Proxmark-antenna just the right tweak to catch all modulations.
If you have better suggestions, don't hesitate to contact me, so I can update this section.