Restriction enzyme digestion?

It's been a while since I did restriction digestion, but if you have good quality enzyme, it should cut within an hour. SmaI digests at 25C, so make sure to get the right temperature (most other enzymes cut best at 37C).
Maybe your lab has a batch of old restriction enzymes, and so they might not work that well anymore.

Other things to remember is that your input DNA should not be more than 10% of your digestion volume, and to get an optimum buffer.

It is probably best to do a second digestion, in a different tube, with a standard plasmid that you know has 1 or 2 SmaI sites, as a control.

Good luck!