In most cases, that's it. Except for a brief period in the 20th Century, there is and was no media or governmental interest. There is no one to take the report.
Therefore the rate that reports has varied tremendously over time:
Variation in Reporting Rate According to Place
A glance will show that most of these differences are probably not due to the actual rate of occurrence of the phenomenon, but instead the level of development and type of government and press in the countries. There are also evident differences due to the population density of the countries.
The density effect has been noted before. UFO's are seen much more commonly in sparesely-populated areas than in densely populated areas:
Variation in Reporting Rate According to Time
During the period of intense media and government interest in UFO's (1947 to 1967), the US Air Force used its (then) substantial base network to take reports and write them up in a standardized format. During this period there were also substantial report libraries accumulated by the private NICAP and AFPRO organizations in the United States, the researcher Jacques Vallee, and other private organizations in Canada, Europe and Australia.
The official Condon Study of 1969 ended the US government program of taking reports. Researchers at the private CUFOS group managed to acquire the final government-sponsored report database on mainframe data tapes. This was maintained by researchers at CUFOS, incorporating data coming in from the other private organizations and publications.
In 1977, disaster struck. Baby-boomer film directors, who grew up during the heyday of saucer sightings in the 1950's and 1960's, started making films. Star Wars and Close Encounters of the Third Kind opened up in theaters and became the most popular movies ever made up to that time. This unleashed a flood of high-budget, big-box office films covering multiple present and future universes teeming with spacefaring extraterrestrial life. It also brought to mass media attention the vast lore of UFO sightings.
This fantasy universe was so much more interesting than the real one that funding and public interest in actual space exploration and actual UFO studies totally collapsed. What once seemed a possibility was now reduced to a juvenile fantasy. Funding for NASA nearly dried up, and contributions and interest in actually studying UFO's almost ended as well. In 1982 CUFOS had to pull the plug on their UFO database.
The period from 1979 to 1994 is a black hole in UFO recording. Based on opinion polls, UFO's were being seen at the same rate as ever. But the press, the police, the military, and the government were not interested in taking the reports. A few enthusiasts soldiered on. But there was no central collation of data.
In the mid-1990's the Internet emerged. This made it possible, at low expense, to collect UFO reports without press or government involvement. Several web sites emerged for this purpose, the most used being nufor.org and mufon.com. Of course this was much less filtered data than that manually collected by governmental or enthusiast groups earlier. The ease of making a report (as opposed to being startled enough to go to the trouble of going to the Air Force, police, or press) meant a lot of trivial reports, a lot of hoax photos or kids horsing around stuffed up the inboxes.
Final Conclusion on Data
The data in the exisiting databases has many faults. They are not a random sample of UFO reports across humankind. There is very little data for the developing world - the Middle East, Africa, south and east Asia, equatorial portions of Latin America. There is virtually no data for eastern European countries during the period they were under Communist regimes.
Any study of the cases has to take all of this into account. There are certain things that can be deduced from the data as it is. Other things (such as actual rates of occurrence of sightings over space and time) cannot be deduced without limitations as to time or place.