Why is Diels–Alder adduct exo when we leave it for a while?
As you probably know, the Diels–Alder (DA) reaction is reversible — the easiest example is the reaction of cyclopentadiene, which polymerizes at room temperature, which — if needed as monomer — has to be cracked just in advance.

In terms of the selectivity you observed, the endo-oriented product of the DA reaction often is the kinetic one, the exo-oriented product the thermodynamic preferred one.  Which is, if the rate of a DA reaction is to be increased while still obtaining endo-preference, an increase of the reaction temperature is skipped in favour a Lewis-catalysis, addition of a perchlorate, or high (external) pressure.