Project cannot be created because you have exceeded your allotted project quota.

We are using the Deployment Manager to create projects from a dedicated Project for DM under an Org Account.
From time to time we keep hitting the following error:

```
project cannot be created because you have exceeded your allotted project quota.\n type:
message: '{"ResourceType":"cloudresourcemanager.v1.project","ResourceErrorCode":"429","ResourceErrorMessage":"The
location: /deployments
```

We have tried changing the Service Account and this doesn't go away unless we switch to another project (or create a new one).
This is strange, as under the 'Quotas' we don't see anything that has exceeded the cap.
Could you please help?