Well you did say you started your DJ 6 days ago; that's pretty new. And yes it's normal for beginners, unless you have the natural ability to recall dreams without effort. I'm 100% positive experienced dreamers in this forum who have been practicing lucid dreaming might also have problems recalling dreams at times. Don't worry too much about not remembering your dreams at the beginning of your dreaming journey.

Your techniques are fine as far as I know. Perhaps your mantra is not strong enough? I believe mantras are more successful if said with full confidence imbued in your body, not just rambling on and on without much heart.

Maybe you can try this technique I found here. Basically you write down your events in the day, no matter how mundane it is. It somewhat clears your mind before sleep and somehow make way for dreams.

Just take it slow, be patient, and results will come to you. I have problems recalling my dreams too, so you're not the only one. All the best!