It is theoretically not the bullets fault as someone has mentioned. That bullet is designed to withstand velocities north of 4000fps and a factory round put together for a very standard twist .223 like the OP mentioned should not provide an issue.
Foxes are squishy and while they take a bit more killing than a crow, they are not exactly difficult to kill. It could be a batch of bullets with thinner jackets than normal that might be failing to withstand the velocity/twist combination but the actual bullet itself should be suitable for your application.
When you say you have had some runners, are they going 20 yds and falling over or are they running off to god knows where and not found? If the former, then I would say that can happen. If you shoot enough, they don't all fall over dead on the spot. Most do with a good round and a sound placement but it is not possible to replicate the same outcome each time.
Handloading will allow you to tune a bullet to your rifle but it will not make the bullet anymore or less suitable for the task. I use a .222 for most of my fox shooting. I don't know how many I have shot but it is certainly well in to the hundreds, maybe more. I bought a couple of boxes of Sako 50g SP's with the rifle just to sight it in and get familiar with it. The second box I shot entirely at foxes and I recall one making it nearly 30yds to a hedgerow despite a solid shot placement just behind the shoulder as it stood broad sideways. The others all fell over on the spot.
I then reloaded for that rifle and have shot mostly either 50g Vmax or 53 Varmegeddon through this rifle. Again, most fall over but both bullets I have seen the odd one leg it before blood loss sees it cartwheel over 20, 30, 40 yds from shot site. Milage for others might vary but if you shoot enough, you will see some oddities.
I would not be changing to a SP personally. Highly frangible bullets are good for fox shooting. Anything north of 50grains doing north of 3000fps is generally going to result in the fox having a bad day. Keep shooting and keep putting the bullet in the right place. It will even out.