A couple of common causes are:
VAT journals take the default Bank and Vat Control accounts from the currency record and if these are not changed when the journal is posted, a mismatch can occur.
New Analysis records also take the defaults when they are set up, with the same effect as above
To troubleshoot and investigate this issue:
Note: The guidance in this document works on the assumption that Sub Ledgers or Entities are defined as Nominal Category 1.
In the search criteria enter the Entity (your Sub Ledger)
Select the Year/Period as appropriate from the earliest to the most recent, in order to identity when the Trial Balance went out of sync.
Make a note of the Year and Period which the balance went out of sync.
Click Financials, Enquiries, Nominal Transactions
In the search criteria enter the Entity, Year and Period and then click Search
All the transactions for that Entity and Year / Period are returned and should balance (Debits vs Credits) to 0, although it is likely that they do not.
To export the entire transaction list to Excel, click the cogwheel icon and select Export to Excel (All)
Within Excel, add a new column which sums the Total of Credit and Debit for each line, resulting in a positive or negative number.
On the Data tab, use the Subtotal feature to group by the Header Line No and Sum by the new field in the new column you've just created. Now you can see which lines do not balance (possibly due to miss-postings between sub ledgers.
Click Financials, Enquiries, Trial Balance Summary