Hi David,
We can not advise you why you cannot use the losses, there are several reasons this might be such as, type of income or type of business.
Some types will need to use third party software to have access to all losses .
Please review guidance below to see if your eligible for losses.
CTM04000 - Corporation Tax: trading losses: general: contents
If you still require assistance please seek help from an accountant.
Thank you.