I use Ilford Delta 100 for most of my B&W work and develop in a Jobo CPP2.
The developing times I use alter very little, no more that 20sec. depending on the contrast of the scene. I expose Delta 100 @ asa 125 not 100.
I use one temperature, that recommended by Ilford; 20 Deg. 'C' or 68 Deg. 'F'
I am unable to comment on your developer times as I use Aculux 2 only, which is a one shot developer for recommended a dilution of 1:9. With this soup my time is 9 minutes +/- 15 Sec. I get a full tonal range.
Nevertheless, your time for the increase should be about right to get a good range of tone.
Have you checked all other parts of the developing operation such as dilution and agitation?
This combination is used by me for all formats from 5X4 to 35mm, I have never found just cause to change it over the last ten years or so.
I think it wise to make a real effort to get your developer temp. spot on 20 Deg. C, and use that as a sort of control sample for any other alterations.