Do you have chickens / eggs also? If you have them, you might want to consider putting a few of those in (either just cull them or time to hatch with the turkeys), seeing how/if they develop might help give you an idea if it is the incubator or if there is something going on with the turkey eggs. There are also thermometers that record high-low temps, if you are not sure about your incubator one of those might be a help.
