I rarely get precum myself. Nothing really wrong with not getting it, it doesn't provide any sensation. Just provides some lubrication, but the woman usually produces more than enough.

The only time I get it is when I'm highly aroused and not brought to orgasm for some time. Usually only during foreplay, extremely rare during masturbation.

The only thing in the diet that I'd think would affect it is your hydration. If you get enough water, that's about it.