Checkout can not be successful in Axure8.1.0.3388 for Mac

Env:
User A with MacBook Pro(Axure 8.1.0.3388 for Mac)
User B with MacBook Pro(Axure 8.1.0.3388 for Mac)
We use a SVN server for team working.

After user A “checkin” something,user B cannot “checkout” anything even the pages which user A did not touch(no any prompt such as “checkout failed”)

Thanks so much if anybody can do me a favor.
Thanks a lot!