TQSL maintains a record of QSOs which have been signed for upload to LoTW. If a QSO is signed but is not subsequently uploaded, TQSL will flag an attempt to sign that QSO again as 'previously signed'. For a small number of QSOs, those can be individually re-signed for upload, but if there is a larger error, it is better to ensure that TQSL and LoTW have the same set of "already uploaded" QSOs. The "Syncronize upoad data with LoTW" entry on the help menu can be used to correct any discrepancies.
Your computer must have access to the internet to perform this operation. You must have a set of login credentials for LoTW to be used to download your QSO data from LoTW.
Begin this operation by selecting the Help menu's Synchronize upload data with LoTW... command.
Waiting for TQSL to process the request will cause "TQSL is not responding" messages, which should be expected and ignored.