I own a Discovery 645 and don't have any issues with the charging cradle. Though I have to push it really down to get it to vibrate, it is not a issue for me that inserting the headset does not charge instantly like turning on the red light.
I sell Discovery 655 on ebay and have sold quite a few and only had 1 issue with the cradle. When my buyer used the Battery adapter to charge the headset, it heated up the cradle and scraped the AAA battery and most of all did not charge the headset. Though I have to replace her unit, I just had to check it myself by buying one from my stocks to find out what was really the issue. I guess a Seller's responsibility to be able to find out really what the problem is and how it came about.
And here is what I found...
If you do not align or place it together the CORRECT way, it does scrape your battery and it does not charge the headset. Even though you see that it fit snugly making you think that it was the right way, you might have done it wrong. Co'z the one I got for myself, I scraped the battery and was not able to charge the headset as like my customer. Took out the battery from the adapter, realign in the other way (use notch size configuration) and .. WHALA... it charged.
I have talked to my distributor in regards to this issue about the charging cradle and was informed that they didn't get any report in regards to any Plantronics cradle charging adapter. I know that my post might not really be the same as your issues.. what I would suggest is return your purchased item and have them change your unit. If you have just got it recently, it shouldn't be a problem. Sellers usually covers 14-30 days return policy.