haha yeah this is happend to me a few times about the invisible wall i mean. If anything it might be a glitch but wouldnt count on it cause by now EA would of fixed it. what sucks about it is even when you click on the DB to tackle is that he wont even move to tackle and they end up gaining 5+ yards until you get safety help.
And about you MLB getting stuck in the mud. What usually happens is that TE have a high rating in speed and acceleration so most line backers will get burned not only cause of that but because mlb lack good ratings in man coverage. so even if you are in man cover and you have a fast linebacker most will get burned because of the low ratings in man cover. MLB usually get stuck in a play action. hope this helps