Children in grades 1-3 who struggle are missing some basic skills that can be remedied using the right approach and the right tools. The BIG 3 tools are Visuals, Story, and Body Motions. All the recommended resources utilize these tools.
The basic skills that are needed are:
Basic sounds - most especially their vowel sounds.
Knowledge of high frequency/sight words is critically important.
Language patterns/sound spelling patterns will help them figure out unknown words.
Addition & Subtraction math facts known instantly and fluently.