20 microns might be thick for slides. try to reduce to 14-16um then it will work. if you want think sections make them free floating sections and mount after that.

Thanks, Sreenivasa!

I´ll try to cut then thinner and I´ll be back with the results. 

I didn´t think about that, actually I thought that they were too thin to be mounted. 

Have you tried to do a free floating staining and mount the stained slides?

I´m sorry, I should have said what I´m staining the sections with: I´m using H&E, so I don´t think I can use free floating... Can I? It´s just for antibody staining, right? 

I think it is possible. I recommend using an endpoint staining hematoxylin like Mayers, to avoid extend washings.It is worth a try.

With which agent you are coating the slides? Assure your coating process to avoid loss of sections during staining process. If possible try to cut sections less than 10 micrometer. I am working on reproductive organs and used to cut sections of 5 micrometer and it gives better staining results. Avoid excessive washing and shocks during transfer of slides from one solution to others. 

Santosh brings a good point. What HE-protocol do you use? Sometimes too strong acid or alkaline solution or dehydrating reagens can lead to loss of sections.

You could try using Superfrost Plus slides which are specially coated for extra adherence.

Also if you are stretching the cut sections  on a water bath  before mounting onto the slide - keep the temp of the water at 38C and wait for 30-40 seconds for all the folds to stretch out.

Just curious as to why you need such thick sections.  Normally H&E's are cut at 4 microns 

Good luck

Why are you cutting sections so thick? 

If it is possible, decrease the thickness. After pick them up from water bath, dry out extra water from slide with a cotton, put them on heating plate with lower temperature about 38-40 for longer time. Also you can use chrome alum or or poly L-lysine solution (0.1%) as adhesive. it could even be possible to use egg white to adhere tissue sections on slides. 

Good luck

I do not know which fixative you used. However, if they are well fixed they will not fall off. Secondly, you should after sectioning free air dry your slides and prefix them in hot oven at 60°C before staining process. Use superfrost slides they are good. Depending on the period you want to stain, they should be in the oven. The thickness matters about what you are studying the tissues for, maybe white matter gray matter architectures. But if you are looking for cellular architectures, you should have thin sections. Again the use of H & E is good. Currently i am doing some and they are beautiful.