The Sinnoh League has gotta be the most blatant roadblock for Ash compared to the others.
Reasons why Ash Loss to Ritchie
- He wasn't very skilled
- His Charizard was disobedient
- His team was weak
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Reasons why Ash lost to Harrison
- Still lacked skill
- His pokemon weren't strong enough
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Reasons why Ash lost to Tyson
- His team wasn't strong enough
- He didn't use reserves (Charizard)(Snorlax)(Bulbasaur)(Squirtle)
- Still lacked skill
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Reasons why Ash lost to Tobias
- Tobias's team were a lineup of legendaries
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Reasons why Ash lost to Cameron
- His team wasn't strong enough
- He didn't use reserves (Charizard)(Sceptile)(Snorlax)(Infernape)
- He wasn't very skilled
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Reasons why Ash lost to Alain
- Alain's Charizard in ME form was around E4 level
So as you can see the reasons given out for most of the league losses had a lot to do with Ash himself and just needing to improve.All of the league rivals showed up before the league and befriended Ash,I wouldn't count Tobias as a league rival since he was introduced in the middle of the league and only met Ash one time.
His loss at the Sinnoh League was a major screwjob since he didn't do anything wrong,he had enough skill as a trainer,his team was good,he used a good majority of his reserves,and he defeated arguably the most skilled trainer in the entire series (Paul).The writers couldn't make up some Harrison/Tyson type of trainer to try and defeat Ash since they'd lose so they came up with a trainer that owned legendary pokemon,that's the lowest of the low.
Could make a case for the Kalos League being another road block because it fully doesn't make sense for Alain to compete at a pokemon league when his real goal was to become the strongest ME trainer in the world.But it wasn't as unfair as the one at the Sinnoh League,Alain was at least down to his last pokemon as opposed to Tobias who was only down to four.Ash could have came even closer to defeating Alain had he used his strongest reserves (Charizard)(Sceptile)(Infernape)(Snorlax) along with Pikachu/Greninja,have Alain go through them,wear his Charizard out and save Greninja for last.