Missing transactions after migrating from QB Desktop to QBO.

I would recommend to open a new blank QBO account and convert your data again from QB Desktop.

https://quickbooks.grsm.io/US

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Compare both QBO accounts and utilize the exporter tool to export the missing transactions from your old account and import them into the new account.

https://transactionpro.grsm.io/qbo

Another option, purchase a 3rd party migration service if required. 

I understand this isn't an easy process for you, @HM Golf.

I'll share some information on why transactions are missing after migrating to QBO. Also, I'll guide you on how you can fix it.

You may notice some differences on how the data appear when you migrate data from QuickBooks Desktop to QuickBooks Online. Some of the transactions don't move to QBO completely. To know more details about it, check out this article: Learn how features and data move from QBDT to QBO. 

In the meantime, I suggest uploading the transactions manually. This way, you can get back into business with all the transactions you want to reflect from your account. Let me show you how:

For accounts connected to online banking:

1. Click on Bookkeeping then hit Transactions then choose Bank transactions, or navigate to Bank transactions.
2. Choose the blue tile for the account you want to upload the transactions into.
3. Press the Link account ▼ dropdown and then hit Upload from file.
4. Choose  Browse and then click on the file you downloaded from your bank. Then hit Next.
5. From the QuickBooks account ▼ dropdown, choose the account you want to upload the transactions into. Then hit Next.
6. Follow the steps to match the columns on the file with the right fields in QuickBooks. Then hit Next.
7. Choose the transactions you would like to import. Then hit Next.
8. Click on Yes.
9. Once ready, click on Let's go.

For accounts not connected:

1. Click on Bookkeeping then Transactions then choose Bank transactions, or navigate to Bank transactions.
2. Choose Upload transactions.
3. Choose Drag and drop or select files and then click on the file you downloaded from your bank. Then tap Continue.
4. In the QuickBooks account ▼  dropdown, select the account you want to upload the transactions into. Then select Continue.
5. Follow the steps to match the columns on the file with the correct fields in QuickBooks. Then tap Continue.
6. Choose the transactions you would like to import. Then click on Continue.
7. Tap Yes.
8. Hit Done.

In case you'll experience any trouble, here's how to fix errors while uploading.

Additionally, same with what @Fiat Lux - ASIA  mentioned, you can visit the Intuit Maketplace to review available third-party importer application you can integrate with QuickBooks.

After uploading, you can match and categorize transactions to put them in the right account.

The Community is always ready to back you up. Keep in touch for any additional concerns about migration or QuickBooks in general. Take the best care!