I can rule out the quench, Parks #50 is much too fast for an alloy steel, although 5160 is one most likely to survive it. I would first look to proper solution. 1525°F is spot on for this steel. My standard soak for an alloy is 10 minutes, but that is with faster heating equipment than an oven. To trouble shoot solution issues, first increase time, that is your "fine tuning". If you get out to 25 minutes, time is not enough, then you move to bumping up temperature, that is your more powerful "coarse tuning."
Carbon steels should not need normalizing and other treatments to achieve good solution, and alloy steels shouldn't any more than stainless. But sometimes chromium bearing alloy steels will be deeply annealed for free machining and a quick soak at the normal temperatures may not be enough. Carbon in solution = a reinforced matrix, and at room temperature this means steel that is harder to machine. Carbon out of solution = a soft ferritic matrix, which means soft, ductile metal. But where the carbon goes when not in solution is into carbide. The deeper the anneal, the larger and more widely spaced carbides, so when it comes time to put that carbon back into solution it takes more effort to strip it off the carbide and spread it through the iron. With carbon steels, this is not a problem, iron carbide (Fe3C) starts to dissolve at 1335°F. But alloy steel will include much more stubborn carbides and the alloying elements presence will slow the speed at which the carbon moves. So I have never seen a piece of 10XX that was 'carbon locked" but I have seen a few bars of steel that got that way with rather modest additions of Cr.
Also, from reading your post where it includes the water test, gives some other clues. Be aware the the maximum you should expect from this steel is 62 HRC ( it makes great choppers but disappointing hunters and other fine edged knives), but all the same, you should have gotten to around 62.5 or even 63 with a water quench. So there is still something going on. As has been mentioned, you do want your quench tank right next to your heat source, but the timer doesn't really start until you hit the oil. Yes you can take too long to get to the quench, if the blade recalesces (goes dark and then begins to glow unevenly) you will not get proper hardening, but it takes a surprisingly long time for that to happen in air.
You want your AAA, and that is what 5160 wants, to be at round 130°F, get the 1525°F blade entirely below the surface without too much delay, and AGITATE! Move the blade in the oil in a cutting an stabbing motion; just holding it still in the quench is a common source of warping or lower HRC readings. Keep it in the oil until you can comfortably hold it in your hand, above 200°F you are not done quenching with most steels but 5160 should be, but...
Clean up the surface to be Rockwelled with a 220X and then a 400X belt and take you test. The 220X will insure the removal of scale and decarburization, and the 400X is necessary for a truly accurate HRC reading. Less than a 400X finish can give you lower readings.
I can't tell you how much heat treatment troubleshooting I do for other makers and industrial clients and the the two most common causes of lower HRC issues are - decarb and improper solution (and in that order).
