Some students in 4th grade who can do multiplication problems involving one-digit multipliers get stuck when they encounter others involving two-digit or three-digit numbers multiplied by a one or two-digit multiplier.
I've found that by first giving students problems that require no regrouping and going over all the basic steps helps. Then I give them problems that require simple regrouping where the products are two-digit, three-digit, or four-digit numbers.
The same goes for working out division problems: I first give students problems that require no regrouping and that show a two-digit or three-digit number divided by a one-digit divisor. And after they master doing these problems we move on to others requiring simple regrouping.
I make sure the quotients in the easy problems are two-digit or three-digit numbers with no remainders. As they learn the basics doing easy problems, students having trouble (or fear of) math gain confidence using their skills.