This usually means that the company does not provide documents that were created before activating your connection on Doccle.
In that case you will receive your future documents, from the first document following the activation, but not the documents from before that time.
You can always request duplicates of older documents from the company in question (or from the employer, in the case of salary slips).
Documents that you have not received on Doccle but by e-mail or other channels can be uploaded to your Doccle account for free. Do you have paper versions? You can scan this with the camera of your smartphone and then upload it via the Doccle app.
All necessary information can be found in this article: How can I upload a document to my account myself?