Suppose that $C$ is a Grothendieck site, and $\mathscr{X}$ is a stack over $C$ (which is NOT equivalent to a sheaf). Let $$\pi\_{\mathscr{X}}:\int\_{C} \mathscr{X}\to C$$ denote the associated fibered category. The underlying category $\int\_{C} \mathscr{X}$ carries an induced Grothendieck topology such that $$St\left(\int\_{C} \mathscr{X}\right) \simeq St\left(C\right)/\mathscr{X}$$ where the latter is the slice 2-category over $\mathscr{X}$.


Now the functor $\pi\_{\mathscr{X}}$ induces the functor $$\left(\pi\_{\mathscr{X}}\right)\_{!} :Sh\left(\int\_{C} \mathscr{X}\right) \to Sh(C).$$ It also induces a $2$-functor $$\left(\pi\_{\mathscr{X}}\right)\_{!} :St\left(\int\_{C} \mathscr{X}\right) \to St(C)$$ by taking the weak left Kan extension of $y \circ \pi\_{\mathscr{X}}$ along Yoneda, where $y$ here denotes the Yoneda embedding $C \to St(C)$ of $C$. Under the equivalence $$St\left(\int\_{C} \mathscr{X}\right) \simeq St\left(C\right)/\mathscr{X},$$ $\left(\pi\_{\mathscr{X}}\right)\_{!}$ corresponds to the projection $$St(C)/\mathscr{X} \to St(C),$$ since this is weak colimit preserving and agrees with $\left(\pi\_{\mathscr{X}}\right)\_{!}$ on representables.


This implies that $$\left(\pi\_{\mathscr{X}}\right)\_{!}\left(1\right)\simeq \mathscr{X}.$$ But the terminal object $1$ is a sheaf, so we should have $\left(\pi\_{\mathscr{X}}\right)\_{!}\left(1\right) \in Sh(C)$, but we should also have it equivalent to $\mathscr{X}$ which is NOT equivalent to a sheaf. What am I missing?



Why are sheaves not preserved in this case?
Here's what's wrong:


The inclusion $j:Sh(C) \to St(C)$ does not preserve colimits. Notice that $j$ has a right-adjoint given by $\pi\_0,$ at least making sheaves reflective. To see that $j$ does not preserve colimits, take for instance the colimit of $\pi\_\mathscr{X}$, first by composing with the Yoneda embedding into stacks, and second by composing it with the Yoneda embedding into sheaves. The former is $j$ composed with the latter. If $y$ denotes the Yoneda embedding into stacks, the former is the colimit of $y \circ \pi\_{\mathscr{X}}$ which is canonically equivalent to $\mathscr{X},$ whereas the latter is the colimit of $\pi\_0 \circ y \circ \pi\_\mathscr{X}$ which is equivalent to $\pi\_0(\mathscr{X})$. 


