Hello!
As Flieger JG1 already said there were several variables. Among them were:
Time of the year:
There was less fighting in the air during wintertime. Cold weather, storm, snow etc. did not only reduce the numbers of the own flown missions, it also often was the reason that the enemy sent no aircraft on reconnaissance/artillery spotting/escort missions. Fewer enemies in the air meant fewer possible victories.
Chain of supply of material (aircraft and spare-parts):
Even the best German fighter units were rarely fully equipped with men and machines. Often there were not enough aircraft due to crashes/losses in combat/repairs. Having a spare aircraft was mostly a pure luxury and so the pilots often had to borrow an aircraft from comrades or they simply had to wait on the ground until a serviceable aircraft was available. In Germany there were considerable problems with the delivery of spare parts and engines what surely influenced the operational readiness.
Chain of supply of (new) pilots:
Occasionally even the best fighter units were not fully equipped with pilots. Some were on holiday, some were ill or were injured in combat or a replacement for a killed/wounded man had not arrived (In this connection it would be interesting if the “Spanish flu” had any influence).
Quality of the members of the unit:
For example during the summer of 1918 a German ace (Loewenhardt?) complained that the fighting and flying qualities of his men were very poor. Other units had the advantage that they had at least some experienced pilots on their list (that does not necessarily mean aces) so that within every flight there were one or two men with the needed “spirit”. An ace surrounded by beginners needs some time to “form” his unit into the desired operational readiness.
Availability of fuel and suitable lubricants:
During the second half of the conflict Germany faced considerable problems to produce enough high octane fuel for the units along the front. The situation became worse in 1918 – when capturing an enemy airfield a few dozen barrels of British fuel found on it were highly welcomed. Training units had to use a fuel of a rather lousy quality. Furthermore the hot summer of 1918 caused problems in those units equipped with rotary-engined fighters (the Voltol Ersatz for the not available castor oil was more or less pretty bad stuff and lead to a lot of additional maintenance work).
Types of own available aircraft and aircraft of the enemy:
Those units having the Fokker D.VII in mid 1918 were lucky. Others still had to fly the old Pfalz D.III(a) and Albatros D.V(a). Guess who had a better chance to shoot down more (advanced) aircraft. No all enemy aircraft were of the relatively outdated “F.E.2”, “R.E.8” and Sopwith 1 ˝ Strutter” type in 1917/18. So the type of the aircraft used by the enemy was also of importance.
Tactics of the enemy:
If you have an enemy “coming to you” because of his aggressive doctrine (the British) this is certainly more favourabe for a Geschwadercommander than an enemy who also keeps an eye on the idea not to “waste” his men and material or who has a slightly more defensive approach to air war. Because then you often have to search for the enemy (probably without result) or he will take care to accept a fight only under more favourable for him.
Time of the war:
Certainly the chance to fight again enemy aircraft was best possible during the big German offensives in 1918. Then the enemy had to support the own troops at all cost and they had to risk much more to get urgently needed reconnaissance reports and photos. Furthermore there was the good chance to attack enemy aircraft, occupied with the task of ground strafing.
Location, location, Location:
It was not easy to become an ace next to the Swiss frontier or somewhere in Russia or the Balkans. And even over Flanders there were times when there were rarely any enemy flyer met during the missions or the fights ended without definite results (for example: 10. January 1918, 15. January 1918, 26. January 1918, 1. February 1918, 7. February 1918, 13. February 1918, 22. February 1918, 2.-3. March 1918, 7. March 1918, 19.-20. March 1918, 30. April 1918, 1. May 1918, 12. May 1918, and so on, and so on). On other days there was a long list of often more than 20 victories.
And the worst of all probably was ...
...that the pitiable fighter unit had to fly
"an aircraft that had not produced aces"!
.