Hi,

you can find useful information about proper concentration and preincubation here:

https://tools.thermofisher.com/content/sfs/manuals/td004.pdf

https://tools.thermofisher.com/content/sfs/manuals/mp07567.pdf

You can also try another dye, e.g. GelRed

In addition, you need to check the wavelength of your transluminator in order to excite your sybrgreen at the good wavelength.

 I think you mean SybrSafe instead of Sybrgreen, Sybr green us used for quantitative PCRs. 

You can try to post-stain your gel. Sometimes if you add the dye to the agarose melted when is too hot you might affect the compound, that's why is better to wait until is a bit cooled down (60 degrees). To post-stain your gel, you need to put your gel in a plastic box (not glass!) and use the same buffer that you have used to run the gel + dye (i.e. Sybrsafe). For example, use 50ml of TBE1X and 5 ul of Sybrsafe (1:10.000) and incubate it together inside a plastic box with your gel for 30 minutes. Then check again under the UV lamp whether you have bands or not. Alternatively, ask to another group another dye (as ethidium bromide) just to check whether yours is expired.  

Sorry, you're right Laura, I meant SybrSafe, I always get confused when I name them. I also tried post-stain exactly as you mention, but nothing. Finally today I tried again with ethidium bromide and everything went good as usually. Problem solved!! I still don't understand what happened but now I can keep moving forward. Thanks all for your hints.

I'm glad everything is working again! Good luck in your future experiments :)

I think 1.5ul is too low! For 50ml gel I use 5ul!