If something has been done, no matter how extraordinary, others can repeat it. If it can't be repeated, then either it is not done the right way, something is missing or it didn't happen in the first place.
-- Author unknown
Starting with this hypothesis let's analyze and describe the consequences of the existence of true prophecy. The first consequence would be:
If something can be done, others can be trained, taught or shown how to do it as well. If something has turned out to be possible, it normally can be repeated with consecutive intents often more successful. When a single human can do something without help, machines frequently do it better, or help to be more efficient, more acurate or faster. Sometimes machines can take over tasks completely and work on their own.
Nostradamus used simple means to make predictions. He did not have access to advanced technology based on mechanics, electricity or quantum mechanics, as these were not available in his century. That would mean that if his visions are truly visions of the future, it must be possible to receive them without devices.
Technology has advanced tremendously since the predictions of Nostradamus. Should it not be possible to improve the number and quality of visions by using some kind of apparatus or antenna? With all our available technology and advanced knowledge you would expect that we should have found out by now how Nostradamus did it and reproduce his prophecies with more accuracy. Fact is that we still don't know anything about the physics of receiving visions. It does not seem that the number of seers and their visions have increased since the development of modern technology. It might even be that there is a decline.
Prophets are known to have received visions and prophecy. The expression 'receiving something' points to a passive roll of the prophet, indicating the need of a second, active party, the source that sends the message. The reason that people can't repeat what Nostradamus suposedly managed to do might indicate that the source is missing, the circumstances are therefore not the same. You might have the skill and do everything correctly, without a source there is no prophecy.
It's like sitting on a desolated island and buiding a radio. if you cannot receive any stations, you will not hear any music, which might lead to the wrong conclusion that the radio does not work...
Is there a possibility that modern society with its apparatus and technology adversely affects (or blocks entirely) the receiving of visions?