There are many changes in the dynamics of retail and it is important to align your shop and your suppliers to meet these retail changes. The first step is to check how your suppliers' importance to your shop is changing over time.
If so go to the register sales reports and call up the "Sales comparison by supplier"
I would suggest putting in the previous financial year and comparing it to the year before, on this client I am using an old file so ignore the dates, but the principle is the same.
I prefer to look at percentage as it shows trends.
Then I press view report; I get this.
As you can see, there are large changes over time from suppliers. You can also see here changes over time by some other KPIs too. Profit is a good one that I recommend. There are many different KPIs on the other pages too.
Overall it makes a fascinating read.
What I suggest you do is discuss this report with your suppliers to see what is happening, hopefully, they should be able to tell you whether it is you or the market. Together it can help you make better business decisions.