Few factors.
1. You're not comparing apples to apples. Different settings would have very different results.
The aperture, focal length and working distance all can affect it and give you different results. This is true even for several thousand dollars dslr lens. Superzoom compacts will be even more apparent. I know it's inconvenient, but try having a photo with very similar circumstances for comparison.

2. You might have knocked the camera. Then one of the lens goes out of alignment causing it to be blur.
If impact is not a factor, temperature change (day-night cycle) might affect the lens alignment too. Especially when it's a cheap lens and the lens might not be secured completely.

3. Fungus inside. It can grow slowly. I've a lens with fungus. After 5 years and it is still a small spot.

This post has been edited by LegendLee: Nov 13 2017, 03:10 PM

Overusing as in snapping a photo will not make it blurry.
But constantly knocking it or scratching it will cause the image quality to degrade over time.

This post has been edited by LegendLee: Nov 13 2017, 05:22 PM