As Boyd says you need some tools to measure fired and unfired (or fired and sized) cases.
If the ammunition is being fired in a strong bolt action and the cases are rimless, the problem is likely due to over-sizing the cases. Most FL die instructions call for the die to be screwed down onto the shellholder when the press ram is at the top which will allow the sized cases to fit into most any rifle chambered for that cartridge. What is needed is sizing that pushes the shoulder back 0.002-3" which can only be done buy measuring fired cases, then sizing them to move the shoulder back that distance. It requires a little time and fiddling, maybe ten or fifteen minutes, to get the die set where it's supposed to be. First screw the FL die down onto the shellholder, and back it off ~1/2 turn. Measure the case head to shoulder dimension of a fired case with the proper tool (Hornady, Sinclair, and likely others make caliper attachments for this purpose), then size it. The case head to shoulder measurement will likely be greater than it originally was. If so adjust the die down about 1/8 turn and resize the case. Measure again. Keep this up until the head to shoulder dimension is 0.002-3" less than the original fired dimension. Measure another fired case, and size it without adjusting the die then measure this case. If that die setting gives the same shoulder bump, lock the lockring in that position. Your cases should now all be properly sized.
If your rifle is another style action or firing a rimmed cartridge the FL die can be adjusted in a similar fashion, but you may need to size the cases another thousandth shorter to assure proper feeding and chambering. If the rifle is a military rifle firing a rimmed cartridge (.303 in an SMLE or 7.62x54R in a Moisin Nagant) you'll notice that case shoulders are blown well forward of where they were on the unfired rounds. That means neck sizing only or a custom sizing die, and likely shorter case life.
If you're having case failures with neck sizing there is likely a problem with an oversized chamber. Neck splits are common on cases fired in factory chambers multiple times, but body splits and head separations aren't if cases are neck sized unless the chamber is WAY oversized.
Some more information or maybe pictures of case failures would be helpful.