Although not having worked on a 300, I have worked on the 100 and 400 rifles and suspect they are basically the same. Gut feeling is that it could be the probe O ring may have failed and stuck? But in any case I think you are going to have to remove the barrel and breech block assembly ... this will have the cocking lever / probe with it. You can then inspect the loading sequence to see what is going wrong. But care: when you remove the barrel / breech take care not to lose the transfer port seal.
I am sure there will be a guide on the net somewhere .. or more detail will come from other people on here. There will be no need to empty the rifle of air if all you are doing is removing the barrel / breech assembly. Note that the 310 may be a tad more complex due to the mag indexing system.
If you fail to find a problem here and the loading probe is moving freely then I suspect the hammer area. Maybe the hammer is sticking. Again it is a fairly easy strip down but if you have never done anything like this before then I advise getting some guidance in case you upset the balance between components.
Now over to the experts:
Cheers, Phil
Above beat me to it ....