Hi there, sales-suppbroker.
We'd want to make sure that all transactions from your Shopify account are downloaded in QuickBooks Online.
Althpugh, I'd recommend reaching out to Onesaas to check why the sync is only getting data from May 2021. Go to this link and proceed to the App information section under the Overview tab to get their contact details: https://app.qbo.intuit.com/app/qboAppCard?shortName=quickbooks_online_with_shopify&queryID=eb9491579....
In the meantime, check out these articles about managing apps in QBO for additional guidance and reference:
Let me know if you need anything else. We're always here to help.