What's the operating system of your PC? Try to download the tool here. If only PA4010 works abnormal but extenders works OK, you may replace PA4010....

If you take adapter B back to the same room as adapter A, how about status of powerline light(middle one)? If they are not on, try to pair them again. But you may have to reset them with the utility...

That's so strange...It seems that your PA4010 cannot maintain a stable powerline connection for some reason when it is placed near the router. Can you turn off power-saving on PA4010 and test?...

V2 is old... The TP-LINKdriver of WN821N V2 for Windows 7 is available here. It cannot install the driver of V4. http://uk.tp-link.com/support/download/?model=TL-WN821N&version=V2#tbl_a Chipmaker's...

What's the IP address of time capsule and WR840N? Can you try ping the time camsule's IP successfully when PC is connected to WR840N?

If all other clients are working OK, it should be that PC's issue. Update driver of wireless card installed on that PC.