The first step to take is to make sure that you have completed step 1, 2 and 3 of the review process, as each could have an impact on the cost basis of an asset.
Step 4 of the review process is designed to help with any issues remaining after you link all your data and classify transactions. This will ensure that anywhere there are still gaps in your data, you can always reconcile this data to your portfolio.
(Click to go to the support article for Step 4: Missing Funds)
You should also make sure that you have connected all your wallets and exchanges prior to completing the review steps.
Any other issues with missing basis would be a result of another issue in this guide, such as missing data from an exchange.