Any comments about interview question

Hello everyone,

I have been working in my current company for a few years. It is a small company but a nice company. People in the company (both employees & managers) are friendly and cooperative, the working hours are flexible, no dress code...
However the things in the company are going relatively slowly. For example, they came up with a R&D plan but one year later the plan has not even been started yet.
Now I am looking for a new job. Actually I did get a couple of interviews and each time I was asked the same question that "Why do you want to change your job?". I don't know how to answer this question properly since I don't want to say any bad words to my current company and I don't want to give the recruiter the impression that I am not a stable emplyoee either.

Any suggestions & comments will be highly appreciated.

Regards,