15 whole boxes and no key date? Wow.
Actually, that's not too unusual based on my findings... I've had 3 "key date droughts" of over 15 boxes (30,000 coins), with one going almost THIRTY boxes (59,320 coins) so far in 9 months (see below).
On average, it takes me 9.33 boxes (1 every 18,667 coins) to find a key date (38 D, 38 S, 39 D, 39 S, 50-D) Jefferson. I've found 12 in 224,000 coins (112 boxes worth) searched... Based on that, out of 15 boxes, you would expect to find 1 or 2, but not finding any would still fit within a standard deviation of what's expected... Now if you went 50 boxes without finding one, I'd be concerned... As you can see from my findings to date below, mine have tended to come in bunches of three...
Feb 18-Apr 18: None (10,180 coins)
Apr 19: 1938-D (among 480 coins)
Apr 20-May 22: None (13,280 coins)
May 23: 1939-S
June 2: 1939-S
June 3: 1939-S (these 3 39-S coins found among 7,580 coins in 11 days)
June 4-June 27: None (21,880 coins)
June 28: 1938-D (among 800 coins)
June 29-Aug 17: None (59,320 coins) - LONGEST "key date" DROUGHT
Aug 18: 1938-D (among 1640 coins)
Aug 19: 1938-S, 1939-D (AU) (these 3 found in 3560 coins - amongst 207 pre-60s in most of 2 LOOMIS boxes)
Aug 20-Sept 29: None (47,340 coins)
Sept 30: 1939-S (among 2,000 coins)
Oct. 1-Nov. 8: None (41,920 coins)
Nov. 9: 1938-S, 1939-S (among 2,160 coins)
Nov 10-Nov 17: None (13,440 coins)
Nov 18: 1938-S (AU) (among 3,600 coins)
So I got 3 in an 11 day span (May 23 - June 3), 3 in a 2 day span (Aug 18-19), 3 in a 10 day span (Nov 9-Nov 18), and 3 others kind of randomly... My best advice is to keep plugging away until you find one... and when you do find one, go back to that same bank and get as many rolls of the kind you found your original "key date" in until they dry up. I believe that half of my "key date" finds are a result of me re-hitting my original point of contact...