Compare:
1.
Ich kann sichdich 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.
Ich sollte dich sehen.
I should see you.
Here I say, that something should happen that is not happening at this moment.
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.