The smallest of the planetary nebulae are sometimes difficult to distinguish from stars in the field of view. What happens is that when the filter is in place, the planetary nebula stands out, but when the filter is moved out of the light path, the nebula is less bright and hides among the field stars. As with any other deep sky object, the larger your telescope and the darker the sky, the more you'll see. A 4-inch telescope is about the minimum size that can be used profitably with a nebula filter. However, the brightest nebulae, such as M42, can easily be seen in a smaller scope without a filter, and even with binoculars, provided your sky is dark enough.
Many diffuse nebulae are relatively large objects, so their light is spread over a wide area.
So while the stated magnitude implies that it's pretty bright, it may actually appear surprisingly faint. Whether they are glorious sights or elusive, you'll find nebulae always interesting.