Controllers will always try to give you descent due to turbulence if you request it but there are a number of reasons why you may be told to standby, such as...
1. You're still in another sectors airspace (especially if you've just been transferred) and we need to co-ordinate descent with the previous sector before we can descend you.
2. There's other conflicting traffic which we need to get out of the way first.
3. We're busy trying to give descents to the other a/c experiencing the same turbulence. Weather and turbulence can really increase our workload. We will try to give you what you want as soon as we can as we all know what it's like to be in the back of a plane in turbulence but bear with us!