Three things that seem to be contributing to poor math achievement:
1. Multiple strategies for novice learners.
Recommendation: Choose an efficient strategy that generalizes easily to (e.g.) larger numbers, teach it well, give lots of practice on it.
2. Too much focus on manipulatives & pictures.
Recommendation: Fluency with abstract symbols sets kids up for later success. Move quickly to the abstract - that's where the majority of practice should be.
3. Not enough practice.
Recommendation: Give lots of practice and then give more. It's how we get good at math.