I can't be certain without seeing your code, but my guess is you did something like this:
oldString = "the Big and Bad Dog";
newString = replace(oldString, "Big", "Small")
newString = replace(oldString, "Dog", "Cat")
The problem is that the third line replaces "Dog" in oldString not the newString created on the second line. You could fix this by changing the third line:
oldString = "the Big and Bad Dog";
newString = replace(oldString, "Big", "Small")
newString = replace(newString, "Dog", "Cat")
Another alternative is to make both changes in one line.
newString = replace(oldString, ["Big", "Dog"], ["Small", "Cat"])
