Maybe you have a drainage problem (such as a toothbrush in the line). Such faults are sometimes intermittent. By all means assure that your waste lines are properly plumbed and free of toothbrushes and other partial clogs.
That being said, the culprit may well be the mandated 1.6 gallon WCs we all must live with…
I’m not an engineer, but I did take two years (each) of college physics and calculus…and I’ve been plumbing for over 20 years. I know that water conservation is a great idea. I also know that deciding how much water to flush is what the math guys call a “Mini-Max” type problem, that is a calculation involving minimizing one factor (water waste) and maximixing or at least achieving another (flushability of waste down the crapper).
If water saving were the only factor (all other things being equal), you should be able to flush your crap down the toilet by just spitting on it….
Obviously that won’t work in the real world. So a lot of smart guys working for the government figured that 1.6 gallons should give an effective flush and save water…
Well, my friends, that’s a noble goal and I’m sure those new crappers do save water…on average… and work more or less well enogh for most folks.
But I know, and I’m sure most of my colleagues here know, that it doesn’t always work that way.
I’ve replaced too many old 3+ gal flushers with the new models. Most do work, but many leave much to be desired, along with a lot of other stuff that should have flushed but remains in the bowl…
My advice to the clients that have to live with this result is: 1) hold the handle down at the end of the flush…most of the new toilets will thus discharge enough “extra” water to get the job done; or, 2) keep the old 3+ gallon flusher… Can’t legally install them in new construction, but they can be “grandfathered” in and replaced with “surplus” like models on the used market. For my environmentalist clients, I point out that an old style crapper can be weaned down to a two gallon flush with adjustments in the tank float valve and a judicious use of bricks and other tank occupying items…and still flush better than a 1.6 gallon crapper on its best day…all other things being equal.