It sounds like the pups have swimmer pup syndrome. One of the main symptoms is a wide almost flat chest along with an inability to walk by the normal age. You can read more on this here:
The solutions recommend are ones that are recommended to try and help pups with swimmer syndrome.
The mat of course should be one that provides lots of traction so I’m assuming that is the case since you said it was a great mat.
There should be some nice soft bedding in the whelping area to help cushion the chest so the chest isn’t flattened even more. Some owners will attempt to keep the pup on their side in order to promote a more normal chest patter. Other methods include cutting holes in the elastic of some socks and making like a tube with holes for the legs to attempt to keep them in a more normal position. I haven’t had much luck with this method though some owners claim it is helpful.
They now make rear leg hobbles that may help. I found this one online.
The hobble along with using a sling to help them remain up might help the best. Since the pups are over 3 weeks of age now, they can regulate their own body heat. Thus you can do swimming therapy to help strengthen the legs as well. Just be sure to keep the pups in a draft free area after swimming and dry them thoroughly.
Give these techniques a try and see if you don’t see some improvement.
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