I have that problem. I cant push sometimes. no matter how full the vault, I simply cant get the stimulus to push!

That leads to the stool contuning to be robbed of water, and then becomes quite dry, and even harder to push. Now and again, I have to lube up a finger with prep H or something and help it find its way out. I was told this IS an MS thing.
I agree, please speak to your MD about this. If its a once in a while thing, its not a biggie, but alla the time needs to be documented.