Delta 400 and DDX for indoors

I've being using Delta 400 in DDX for the last two years (since I started developing myself) and I'm very happy with the results. My only issue is that when shooting indoors, regular home bulbs no flash, I would like to have more contrast in my negatives they look like a gray card. Any ideas?

Thanks
Rafael

Try developing longer, say 20 per cent to start.

johnnywalker said:

Try developing longer, say 20 per cent to start.

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Interestingly, your recommendation almost matches the Ilford recommended time for EI 500 at normal contrast. Will I get more contrast exposing at ei 400 or 500?
Thanks
Rafael

Contrast is controlled by time in developer. Increase the time. By increasing ISO a little, you can increase the time even more, but at some point you loose shadow detail.