There are 4 factors that determine IPI and here are some general tips to increase your IPI;
1. Excess Inventory - this is inventory that you have at an FBA warehouse in excess of what Amazon's algorithms think you can sell. Having inventory that will take months (or years in some cases) to sell in an FBA warehouse can severely hurt your IPI because Amazon does not want to waste its FBA storage space on products that are not selling (and you shouldn't want to waste your money paying Amazon to store slow-selling product).
Try not to send more than 90 days' worth of inventory into FBA. If you have too much inventory relative to estimated and actual sales, your IPI will go down. Excess inventory will also result in increased storage fees so it is good to avoid sending too much inventory to FBA of any single product.
2. Sell Through - this metric is based on how quickly you sell products that Amazon thinks you can sell quickly (unlike excess products that Amazon's algorithm does not think you can sell). This metric can be negatively impacted by things like pricing your product above the Buy Box price so you don't get any sales on a potentially fast selling product.
Try to always match the buy box and keep your product moving. This is also good business practice since a product that is not selling is costing you twice in terms of opportunity cost (losing out on the sale AND losing out on a potentially good product you could invest in if your cash wasn't tied up in a slow-selling/not profitable product)
3. Stranded Inventory - this is inventory that Amazon marks as "stranded" for various reasons (usual things like pricing errors). This is one of the easiest metrics to control because the product is not often stranded and as soon as a product is stranded you can either fix the stranded problem or have it removed.
Always check for and fix stranded inventory as soon as the inventory becomes stranded.
4. In-Stock Inventory -this metric is based on how well a Seller keeps products they've sold in the past in stock. If a Seller is always running out of inventory and/or has many products listed in their inventory that have 0 in stock, the In-Stock Metric will suffer.
If you are selling a lot of one-off products (the types of one time deals from closeout wholesalers that you can only buy once), consider hiding the product from your restock report. You can indicate that a SKU is not replenishable on the Restock Inventory page by clicking View details in the Action column and selecting Hide recommendation. Hiding all SKUs associated with an ASIN will exclude the ASIN from your FBA in-stock rate and estimated FBA lost sales.
You can learn more about your IPI score and how to increase it here: https://sellercentral.amazon.com/forums/t/fba-inventory-performance-index-ipi/397597
Not only will following these tips help you increase your IPI over time but it will also help you increase the efficiency and profitability of your business.