serious decline in sales

I had really good August and September and October started off quite well, but in the past few days sales dropped considerably, and I don't understand why. 

yes

Yes, drastically for me for the last three months. To the point that I am discouraged to add any more images. Videos are still a bit better and reviewed quicker. I contribute to around 7 platforms and Adobe used to be on the top. Unfortunately now is at the bottom of them all. 

Adobe constantly adds assets (more choice for the customer, but more competition to existing assets) and there are more customers using stock (more sales), also because stock is used in some Adobe programs and services like Adobe Express. In addition, there are very interesting subscription models for companies. 

Now, appart from seasonal effects, it may well be that your assets get less visibility, because customers select assets from competitors. The more assets Adobe adds, the more pronounced this should get. 

Estou a sentir o mesmo, o mês de setembro foi fraco no número de vendas e este mês de outubro também não está a melhorar. Não tenho imagens em AI. O portfolio não é muito grande mas envio com regularidade e nesse sentido ia tendo vendas mas agora diminuiu bastante. 

A bad month of August, but really bad! With a resumption of sales around mid-September and which continues upwards.   My portfolio is 80% AI, 20% photos. 

Changes in customer purchasing habits comes with the territory. 

• Are you marketing your content on social media sites?
• Have you created your Portfolio website?
  https://help.myportfolio.com/hc/articles/360036798993-Sharing-Adobe-Stock-Collections-on-Adobe-Portfolio
• Are you adding fresh content regularly?
• Have you been watching trends to see what types of assets people are buying and adjusting your content accordingly?

Not really. 

No.

No. I submit photographs (no AI), and after slow sales in August, I have experienced higher than ever sales in September. October is off to a good start too. If your portfolio includes only AI assets, it's possible that declining sales are a result of the dramatic increase in the number of AI assets in the database which have created a lot more competition.