Practice making the short u sound with a child. Make sure the child is able to make the correct sound. The C stands for consonant and the V stands for vowel. Explain which is which to a child.The CVC rule gives a new reader a clue to use a short vowel sound. I explain it this way. The vowel is all alone in the middle. The vowel is shy and makes a short sound instead of shouting his own name. There are exceptions, but they usually follow a spelling pattern. (find, grind, old, cold, sold, told, wild, child, mild) I've found that vowels alone in the middle of consonants usually make a long sound next to a blend or two consonants together. These seem to be the rule breakers.
Have you printed the flashcards I shared? The words but, fun, and run follow this rule.
Here's a video of me teaching how to blend the sounds and read the words using magnetic letters.
Starfall has Gus the Duck and a word making game you may want to use to practice the short u sound. Starfall leaves gaps between sounds in words and breaks off the first sound instead of trying to blend the vowel sound with the first sound quickly like I showed in the video.