It could be either 1. that they are still a bit stressed from shipping/new home. 2. The food item is so small they don't see or recognize it as a food item. 3. The item is not warmed or moving to their satisfaction or 4. They are just "bad" feeders.
1. I've had snakes that have taken over a month to get settled in before they started feeding and some that feed just a few days after arriving. Best to just leave them be for a week or two before attempting feeding.
2. If these are adult balls and they were only eating 1 mouse a week then they are probably underfed (IMO) I would suggest several mice a week or small rats. An appropriate meal size for a ball is about 15% of their body weight per week. I understand that you don't want to place a new order right now - if you can't find f/t near you, I would suggest picking up a couple of small live rats, pre-killing them and seeing if they will take rats before you order their food. If not then pick up some live mice and pre-kill those. It would work to your advantage to get them established feeding wise and switched to rats - before you order - but be aware that some balls established on either live or pre killed will only take live or pre-killed and won't touch frozen thawed.
3. From what I gather - corns will eat anything that is put in front of them (I don't know this to be true I don't work with corns - but it's what I've heard) the same is not the case for some balls. If you are feeding frozen thawed you'll most likely need to warm it up to about 98* before they will even look at it - some will take it sopping wet off the cage floor once they have been trained to do so, some need to see a little movement in order to strike - and size does matter if it's too small they could pass it up unless they are really desperate.
4. Bad feeders happen - it's just something you have to work with - hopefully you have the resources to do so if they turn out to be not the best of feeders - if the previous owner was feeding these adults only one mouse per week it doesn't sound like they were being fed appropriately or that's all they will take - they may take some working with to get them feeding properly.
Hope some of this helps -- best of luck with them!
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Tosha 
"Nihil facimus sed id bene facimus"

6.34.0 Ball Python (Harry and Fluffy and gang)
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