Just tell them you don't think they're as good as they used to be. Although this may simply be due to you getting better.
When I first started Aikido, my teachers seemed so amazing, after training regularly for 3 years, going to seminars, trying to regularly train certain things and introduce new ideas I realized that they were.... mediocre. Unfortunate, but true.
There's nothing you can really do to make them be better unless they're interested. Even then they may say they are, but not really be interested.
There are many mediocre but don't realise it? I'm not the prettiest, I'm not the best, but I know it works, that's all I have to say on that one....