Children, once they reach the verbal capacity, will let us know and show us signs that they are ready for the next step. If we as parents find ourselves putting the pressure we feel about making that correct time mark onto the child, then the training will appear difficult and seemingly unsuccessful.
That's not to say the parents' heart isn't in the right place, we all as parents want to feel we are doing all we can to help our kids along.
The key to focus and work on first is our kids being able to communicate the words referring to going potty and making that verbal connection. Before this happens it is very hard to get kids to go potty. The moment we parents find ourselves listening to our child talking about going potty is the time to go for it full speed ahead.
Prior to this occurring, the visual sight of the potty or child toilet seat and them hearing reminders about doing that action through key words is enough to get them to start verbalizing it to us first, rather than expecting them to do the act itself. Usually shortly after kids start communicating about 'going potty' they will begin to physically feel more confident about doing the act itself.
Hope this takes some of the stress out, no pressure equals more results, good luck and have fun!