There are a few possibilities, including:

1.) The program was previously installed on the computer. Since each trial is one-time only, reinstalling the program will not grant you a new trial period.

2.) You waited more than two weeks between installing and running the program; the download is a 15-day trial only.

3.) Sometime between installation and that execution of the program, the clock was changed. Many programs will automatically terminate the trial period if tampering is detected, even if the change was harmless, unintentional, etc.

4.) A registry cleaner or similar program deleted a file or entry that is used to determine if the free trial is still valid.

5.) The program simply malfunctioned, be it due to a programming error or random glitch.

Regardless, there's little you can do but contact the developer to report the issue and request the opportunity to restore the trial period.

Sorry.
John