I'm not saying your child has ADHD, but just because someone can pay attention to an intrinsically rewarding task does not mean that they do not have ADHD. Most problems with attentional allocation show up on tasks that are not interesting. Interesting tasks capture attention. People with ADHD have difficulty with inhibition, with keeping their focus from shifting off uninteresting tasks onto ones that are more novel or intrinsically rewarding.
Even if a child doesn't have attentional issues, it still uses cognitive resources to maintain focus on a boring task, and studying math facts is pretty boring by any standard. If the child has a problem with acquiring math facts, making the process more interesting may also make it easier and more successful.
Sometimes working with manipulatives (such as different sizes of Legos), making the math facts into songs, games, or patterns, or incorporating them into intrinsically rewarding activities that require fluency with math facts to win (like card games such as 21, or computer games like Math Blaster, Timez Attack, or Multiflyer) can help increase fluency with math facts without the need to also overcome boredom and devote cognitive resources to sustaining attention.
If the difficulty with retaining math facts persists in the face of practice that should be sufficient, evaluating for dyscalculia might be in order. Remember, though, that this is a rote memorization task, and mastering it may take longer than you are used to seeing it take your child to master conceptual tasks.
