If they are fragments of low molecular weight have you tried increasing the agarose percentage of the gel? This can improve resolution somewhat.  1-3% depending on your fragment length.

https://www.thermofisher.com/uk/en/home/life-science/pcr/elevate-pcr-research/agarose-content-with-tips-and-tricks.html

Run the gel slowly and do a large gel instead of a small size.

Hope this helps

I have used 2% agarose gel and was run under 70V for ~1 hour 30 min