Sampling problems/ stretching

OneL

Hi All. Newbie here. Using a Mikro MK3. I’m having challenges stretching samples to a specific BPM. When I apply the stretch, something goes wrong. If I set the BPM it rarely matches the metronome. Is the problem that I’m not cutting the sample precisely? Most of these samples are from older songs that I extracted from Apple Music into a WAV file, and then needed to trim. Clearly I’m doing it wrong. Any help would be appreciated. Thank you!