https://www.musical-u.com/learn/why-cant-i-tell-whole-steps-from-half-steps/
Distinguishing intervals which are close together, like semitone and tone (a.k.a. whole step and half step / major and minor seconds) is a sticking point for many students as they learn interval recognition.
If you get stuck with a certain pair of intervals don’t be afraid to adjust your lesson plan to work around it.
After improving your ear for other intervals, you’ll find you can return to the problem pair and will find it much easier.
https://www.musical-u.com/learn/why-cant-i-tell-whole-steps-from-half-steps/