This is an issue that I was recently having and couldn't find any solutions from previous posts. I did find a solution and figured I'd put it out there just in case someone else is having the same issue.
The first time I screwed my Mephisto rda in the reader it worked perfectly fine. However later down the line trying to use the smok omnitester it wasn't registering. I found that the rda I was using had a shorter pin than others I had since measured. The pin in the Omnitester got pushed down and didn't spring back causing RDA's with a short post to not get a full connection. The solution I found was to simply find something to fit into the connection of the meter and wiggle or pull the pin back out. After you've done this it should work fine again.
The first time I screwed my Mephisto rda in the reader it worked perfectly fine. However later down the line trying to use the smok omnitester it wasn't registering. I found that the rda I was using had a shorter pin than others I had since measured. The pin in the Omnitester got pushed down and didn't spring back causing RDA's with a short post to not get a full connection. The solution I found was to simply find something to fit into the connection of the meter and wiggle or pull the pin back out. After you've done this it should work fine again.