We have had only an occasional stopped up toilet--relieved by using the plunger. In the past two weeks it happens at least once a day and sometimes several times a day. We live in the country and have a lagoon system. What is causing this and how do we fix it?
Thank you.

We had the same problem with one of our toilets. It was always a crapshoot (no pun intended) to see if it would clog or not.

The problem was that the flush wasn't strong enough. I went out to Home Depot and got a Fluidmaster Toilet Repair Kit ($22.99) and replaced all of the tank guts. Now it will flush down a bag of gravel if we want it to.

I have been told that most toilet issues can be fixed by replacing the tank mechanisms.

It's also possible that someone may have accidentally flushed something down the toilet like a comb. You may need to remove your toilet from its base to find it.

It can be number of things..

I do not know what is "lagoon" system. But I assume you mean septic / leach field sewer drainage system.

First, make sure your septic tank is not full. If it is, have it pumped out.
Then, your sewer pipe may be clogged. If that's the case, have it snaked through the clean out. If you don't have clean out, remove the toilet and clean it through the toilet opening. You could also snake it from the roof and through the vent.
Thirdly, you could have accidentally flush something into your toilet. This object ( hair sprayer cap, etc ) is now lodged in the toilet trap making it to back up.

Milo, Lagoon Sewer Systems,(see image) don't use a septic tank.
Wildridge, Have you augered your toilet? (see image) If not do so. While a plunger will push paper and solids past the obstruction a auger will auger into and either break it up or retrieve it.
I like Milos third explanation
Thirdly, you could have accidentally flush something into your toilet. This object ( hair sprayer cap, etc ) is now lodged in the toilet trap making it to back up.
I also like s_cianci solution.
You may need to remove your toilet from its base to find it. If the auger won't cure it and you decide to pull the toile and go in from the bottom click on back and we'll walk you through it. Good luck, Tom