If there is ANYTHING wrong with the coils or the machine they will find it. I just have to believe there is something wrong with part of the system as you've done a buried coin test, yes it was a fresh bury so no corona effect, but you still should have been able to find it. I just reread your post, you bought it brand new this spring so the check/repair should not cost you a thing! Call Whites and talk with them, they Will be very good to you they might even pay for shipping & insurance in both directions.
I have an XLT Spectrum that I purchased brand new a long time ago and its likely I still do not have the hours on it that you do. I have the 950, the 600 and the 350 concentric snooper coil. IF everything goes well I intend to purchase the 4 x 6 shooter coil this fall. Why? I'm living back in the city and I plan to start using the machine likely in December during the plant shut down. And, I want to see the difference between the 350 and the 4 x 6 DD coils as if there is an edge, one coil is more useful in certain situations, then I'd like to be able to use that edge. Also, I sent my XLT in last year for a check up and a new mounting of the box to the shaft and they went through the entire thing and even updated it as it did not have a ground lead from the box to the circuit board. Sending it in was a good thing as I "know" it is good to go. I feel you need that assurance that the machine is everything it should be otherwise you will not be enjoying yourself, worse yet you will doubt the machine. I also sent my GMT back in as I managed to break a toggle switch. They fixed it and got it back to my really fast, the XLT did not take long for it to be back with me so the turn around service is worth it.
Were you using AA Alkaline Batteries or the rechargeable pack? You might want to switch to absolutely fresh AA's and try it but I doubt it is a battery problem.
Were it I in your shoes I would send it in for the peace of mind.
A little about me: I started out with a White's 6000 D Coinmaster in about 1976. Once I learned how to use it I was in heaven. Then it was stolen and I was without a detector till I purchased the original Fisher Gold Bug. Never found a piece of gold with it. Then I purchased the XLT ~ 1995 and used it some but just never got back into it as I was living in the country and the location just did not work for detecting. In 2007 I purchased the GMT as I was backpacking and wanted to try again to find gold. With only making a few trips into the hills each summer it took me till 2010 to find gold and I've found it every year since then. Not large quantities but finding any gold is a thrill. I sold the Fisher Gold Bug last year for $125.00. That's the way things go sometimes. I found Whites machines do find what we want to find and that once I've learned them I trust the dickens out of them. My 6000 D - I learned how to super tune it and would run it in all metal, I'd get a target, flip the switches, twist the knobs (all while waving the coil over the ground) and before I dug it I knew what was in the ground. It was amazing and I'm looking forward to the XLT doing the same only better. I purchased a second 6000 D from ebay just to have it to play with.
Okay, send yours in, get it back and go have fun! Please, let me know how it works when you get it back!! Thanks.....63bkpkr