I requested a shop because it could not be self-assigned. My request sat there for more than a week. I was tired of waiting and canceled my request. Then, I saw the shop could now be self-assigned. So, I grabbed it.
This was not a case of my shopper rating improving to let me self-assign. I have self-assigned this shop multiple times before and I could/can self-assign other shops for the same client.
If you have a shop request that has not been granted or denied for a long time, you may try canceling it and see if it will then allow you to self-assign.