Sometimes I am trying to download big files, after few times it fails to resume and doesn't continue, why is that? any way I can prevent this from happening?

In most cases it's a problem with expired temporary links. It's been explained more than once, see these old topics for example:
viewtopic.php?f=1&t=18523&hilit=temporary+link%2A
viewtopic.php?f=1&t=18151&hilit=temporary+link%2A
viewtopic.php?f=1&t=17741&hilit=temporary+link%2A

In some cases you can resume such a broken download when you generate a new download link and then use context menu "Change URL" (suppose you are using current FDM 6 build).