My low D note plays up an octave

I've been playing alto saxophone for around a year and a half but had always had a problem where my low D note doesn't play correctly. I can play every note correctly from low F yet, anything lower I feel I have to really slow my air in order to hit it correctly without it cracking and going up the octave. Even an E messes up sometimes. It's almost impossible for me to tongue notes from E to D. Anything lower is practically impossible. And no, this is not a technical problem because I've had other people play it on my sax perfectly. There's something I'm doing wrong and I don't know what.