Here are some resources:

Saxophone – Sax Startup (for beginners)   Saxophone Major Scales

Clarinet – Clarinet Major Scales (2 octaves)   Three octaves    Major Scales In Thirds

Flute – Flute Major Scales (1 octave)   Flute Major Scales (2 octaves)

Trumpet – Trumpet Major Scales

Improvisation Exercises – Improv Shape Worksheet   Improv Rhythm Worksheet

Obvious things to check if you’re struggling:

• Are you practicing enough?
• Are you practicing consistently (six or seven days a week)?
• Is your instrument in good repair?
• Does your mouthpiece work well, or are you always fighting it?