Reed relay driving solenoid "sticks"
Every relay has a life expectancy... they are not infinite. You are saying that at 3 hertz you activate it a few million times, that is a lot of activations for a relay.

As you have not provided a part number, for illustrative purposes, I have used a generic one. The datasheet of your relay should have a graph like this one...



As you can see, in this relay particularly, the maximum life operations it can have no more than a million and a half operations...

That type of failure is normal in relays because the contact material starts wearing away with normal operation, and if you operate at the maximum rated load, as you can see in the table, the relay will have no more than 30,000 operating cycles.

I hope this information is clear enough.