As this is primarily a user to user forum, someone-else may chip in. But please remember that the same symptoms do not necessarily mean the same problem.
Check whether there is any silence at the start of the sample files themselves.
If not then, the best thing to do is
contact Support, via here. There is a link at the top of the Search FAQs page to the Technical Support form, for some one to one investigation and help. Response time is usually within 2 working days (US (East Coast) Business hours, generally), but in the current circumstances please allow some more time.
You will need to reply to the Acknowledgement email and attach the imported instrument (the .st4i (or .st3i) and the .pak files) and the original sample files.
And, to help others who run into a similar problem, please let us know what you did to resolve the problem. Please do not quote directly any correspondence from Support, as that may apply only to your particular circumstances.