I think we'd need a lot more info, or more frequent updates from the OP, to answer this fully.
In the interests of being talkative, I will say that I have heard of more issues with the P30L than the regular P30. It seems to require more breaking in than its smaller brother. Also, while the lighter 115 gr loads might be fine in the P30, with the added slide mass of the P30L you might get some issues. I'd try running some hotter stuff, maybe 124 gr NATO, to see if it still has those issues. Locking the slide back as was mentioned is another good idea. I'm not sure how many rounds you put through it in that session, and it might just need to be run some more. However, if you keep having issues don't be afraid to call and talk to HK.
Know the status of your weapon
Keep your muzzle oriented so that no one will be hurt if the firearm discharges
Keep your finger off the trigger until you have an adequate sight picture
Maintain situational awareness