BSR is relative to other products in the category, so an increase in sales won’t necessarily mean an improved BSR. If your product’s sales have increased, but so have the sales for competing products in your category, then your BSR may not improve. Similarly, if your product is ranked #1 and sales remain steady, your Best Sellers Rank may drop if competitors start selling more than you.
In some categories there are millions of products, so the BSR will range into the millions also. It can be near-impossible to to break into double or single-digit BSRs in those categories. Some sellers will select only their product’s least competitive category to track, and try to get a bestseller in that category alone. This strategy often makes sense, but don’t be tempted to choose an inappropriate category for your product to improve your chances of getting a bestseller, as that would be a violation of Amazon’s Product detail page rules.