Objects are more distinct when viewing straight up because you are only viewing through an atmosphere of about 10 miles thick. This is opposed to an atmosphere 15 miles thick at 45 degrees, and over 100 miles thick near the horizon. A high magnifying telescope is very sensitive to light, resulting in a washed out image or an annoying glare. The effects of bright light become more obvious near cities. Many of the stars seem to disappear near the city horizon.
Moonlight can be another factor.
The harsh glare from a full or bright moon can dim nearby stars and planets. The moon itself is best viewed during the phases near the area dividing the darkness and sunlight. A bank of clouds is impossible to view through. Fortunately these are constantly moving. When stars twinkle rapidly, this is a result of the warm and cool air mixing.