Point sources such as stars have apparent magnitudes that indicate their visual brightness. The light of an extended object such as a cluster, nebula, or galaxy, however, is spread out over its entire area, so you also need to consider the size of the object, indicated by its apparent diameter.
The Pinwheel Galaxy (M33) in Triangulum has an apparent magnitude of 5.5, which would suggest a fairly bright object, but it has a large apparent diameter of 62 arc-minutes, twice that of the Full Moon. This means the galaxy has low "surface brightness" and is surprisingly hard to see.
Objects that have large diameters and faint magnitudes— as many galaxies do—require low magnification and attention to contrast distinctions, while objects with small diameters and bright magnitudes— such as planetary nebulas— demand greater magnification and accurate positioning of the telescope's field of view.
Another factor that will affect the brightness of a deep-sky object is its altitude. If a galaxy, nebula, or star cluster is low over the horizon, its light must traverse more layers of our atmosphere than it would if it were overhead. Your best views will be when the object is high in the sky.