You may have taken a Sleeper ticket, but that does not entitle you to enter a RESERVED coach, unless the TT permits you, and he can permit you only when there is a vacancy.
Note that elsewhere on the blog I have said that Railways "May" allocate sitting accommodation in Sleeper Class/other classes upto capacity.
But that has not been implemented.
Merely...
more... having sleeper class does NOT entitle you to enter the Reserved Sleeper compartment wihout the express permission of the TT.
Good wishes.