4064 is a relatively easy powder to ignite, but the general rule of thumb with most rifle powders is to load to at least 70% of volume at the low end, and under no conditions load under 60% full because pressure spikes can occur.
So my first thought is your primers are under-pressurizing the case for the purpose of sustaining ignition. For this reason, it is not uncommon for a magnum primer to work better with low case fill, and then for a standard primer to work better (as judged by lowest velocity SD) when the case is full or nearly full.
However, you said you had ignition failure at 75% fill. That can be erratic with a standard primer in a case the size of yours, but failure to ignite at that fill level is odd and start to steer me toward a contamination issue.
Try what Otto suggested, and load a round whose case you can fill with the powder without going overpressure. If that fails, see if you can get a capful of 4064 from a friend who uses it and do a visual comparison. Look for your jug to have an oily gloss on it. That's an early breakdown symptom.
Finally, put enough for several rounds in a sealed (tape works) container with as much desiccant as you can fit in and try it again a month later. (Note that desiccating raises burn rate, so test with maybe a 60% full charge to start).