How do I deal with lies about being hungry and wasting time making food and her not eating?

My mother dealt with this when we were kids by serving what had been turned down, at the next meal. If you think that this is deliberate unpleasantness (ie not dementia), try that approach. If you think something is wrong, we need more information, as Geaton says.

Can you please provide more detail: how old is she? Has she been diagnosed with dementia or does she have other medical/cognitive issues? Does she live with you, are you her caregiver? Thanks for any info you can give.