Animation in PowerPoint which is designed to involve or react with timings is not ideal.

I see from your thread you were using a dissolve in animation set with the start after previous setting and 5 second delay. Changing the delay to onClick may well have consequences because of the start after previous setting.

If the previous custom animation does not "appear" to have finished to PowerPoint then it is unable to play the dissolve. This is very much a timings issue.

It would seem the reason why your presentation does not "wait" for you to click is because you have the start after previous setting in your custom animation which tells PowerPoint to start the animation after the previous has finished.

The previous will have finished the moment that you entered the new slide because PowerPoint does not "carry over" animations and so it would read your animation as dissolve in, start after previous. The onClick command has no meaning in this set up because you are overrding it with the start after previous command itself. There is no previous if this is the slide dissolve command and set to be the first transition/ custom animation on the slide.

It would have worked correctly with the 5 second delay because either PowerPoint entered the slide. Paused 5 seconds then executed the dissolve. Or you have altered the order of your transitions. If you have not altered the order of your transitions, then this sequence has never been set correctly. It may have appeared to have worked correclty, as the timings appear correct, but as you have discovered when wanting it to execute with the onClick command it does not execute correctly.

You have to change the start after previous entry, and/ or add a one second "delay" to execute then use the start after previous entry. This will "fool" PowerPoint into executing an animation (the one second delay) and then not executing the dissolve in until you click.

From your description it would seem your problem is you have no initial animation to execute therefore PowerPoint starts after previous because there is no previous and when it has finished would then execute the onClick command.