Possible Causes:
Some grades of toilet paper contain lanolins, dyes, and other compounds that provide certain benefits to consumers (i.e. softness, fragrance, strength) yet inhibit the proper breakdown of the paper in flowing water.

There are also many brands of paper that use raw materials of different types with different breakdown characteristics. For example, most paper manufactured by Fort Howard uses White Pine to create the paper pulp. Other companies may use whatever material is prevalent in their region, i.e. Southern Pine in the south.

To test your toilet paper

1
Put a single ply sheet of toilet paper in a quart mason jar with tap water.

2
Shake the jar moderately for 10 seconds; the duration of a typical toilet flush.

3
The paper that dissolves or shreds most will provide the best flushing performance and less opportunity for clogs to develop.