Have problem with flushing toilet. Sometimes doesn't work.

Do you live around trees? As a former property manager I find that my units that had trees in the vicinity often had roots that made the pipes work in some places and not in other. Worth looking into. Hope it helped.

Do you have children that use that toilet and not the one that works? Or someone who uses too much toilet paper? Or someone who has a constipation problem and has large hard stools?

How much water is in the tank to flush? Here is how to adjust it.... https://www.youtube.com/watch?v=XR4OnCjPg2E

Then I would also get some Roebic Drain Cleaner, mix it with warm water and put it down the toliet before bedtime, and let the enzymes eat the crud while you sleep, flush in the morning and see if it works better.

The other possibility is the toliet handle needs to be replaced, or the flapper at the bottom is leaking and needs replacement. There are videos on You Tube how to do all 3 things. Get parts at the hardware store, take you old one with you cause there are a million sizes and styles.

try adding a little dish soap in the bowl before flushing

Sounds like something is floating in the toilet cavity. Maybe a toy, a partially filled bottle. It's stuck in the toilet cavity and floats. It moves with every flush and sometimes blocks the passage, sometimes not. You would need a toilet auger to pull the item out or sometimes push it through. If this doesn't work you may need to remove the toilet and remove it from underneath.

http://www.homedepot.com/p/6-ft-Toilet-Auger-with-Bulb-Head-59797/100015601

Go to one of the big box hardware type stores and buy a plumber's snake. We had the same problem but using the snake a few times took care of it.