Sunshine31-I know that the “Th” sound is also problematic as well as the final “R”. For example, if a person says, " Ca", for the word car, you might not be able to tell that that it what they meant to say.
Just like if the word was, “There” and you say dare it will not be clear. However, the proper tongue placement for the “Th” sound is a protruding tongue.
Sometimes when people slur their speech it is difficult to discern what they are saying because they lack proper articulation.