Dementia changes over time, and so should your expectations. A strategy that worked beautifully last year may not fit today, and that is not a failure on anyone's part. It means the brain has changed and your approach gets to change with it.
Broadly, abilities decline in stages, from early (forgetfulness and word-finding trouble, often still independent) through middle (more memory loss, confusion, needing help with daily tasks, more behavior changes) to late (significant memory loss, heavy dependence on others, and communication that becomes more about presence than words). The exact path varies by person and by type of dementia, so use stages as a general map, not a strict timetable.
The practical skill here is matching your expectations to who is in front of you today. Expecting last year's abilities sets you both up for frustration. Expecting too little can rob your person of dignity and the things they can still do. The sweet spot is meeting the brain exactly where it is right now, offering help where it is needed and stepping back where it is not.
When a once reliable approach stops working, do not push harder. Get curious. Ask what has changed, and adjust. Flexibility is not giving up. It is skilled caregiving.