FPS Cuts MP4 Short

[atkinson40]

I'm editing a Tai Chi MP$ that I want to slow down.  I'm doing this by changing the FPS on filters, but get the end of the video cut of about 10 seconds short??  Any idea why and how to stop it?  Thanks kevin

[AQUAR]

Which version of Avidemux are you using (version 2.6.x it seems)?
What codec is used for the video inside the MP4?

Try using the frame based version (V2.5.6) with the frame rate changer option.