Compare:

1. 
> Ich kann sich sehen.  
> I can see you. = I am able to see you.  

Here I say, that I have a certain capability, namely the capability to see you.

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2. 
> Ich sollte dich sehen.  
> I should see you.  

Here I say, that something should happen that is not happening at this moment.

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3. 
> Ich sollte dich sehen können.  
> I should be able to see you. 

In this sentence, I say, that I should have a capability, that I don't have now, and this capability is to see you.