As of chapter 8, most of Ash's original Pokémon have been shown setting out to look for him (even the ones he received as eggs have been retrieved by their teammates), with the exceptions including Muk, Tauros, Lapras, and Snorlax. The reason these four haven't gone looking for Ash? For logical reasons, they can't find him.
While all four of these Pokémon share the obvious issue of them being fairly large, in comparison to the rest of Ash's team having reverted to their first stages of evolution (the exceptions being Heracross and Palpitoad, and Palpitoad isn't that large while Heracross has the advantage that he can fly), each one also had more specific reasons for staying where they are;
Muk obviously doesn't move very fast and is also not exactly stealthy, so it was easier for him to just wait in Gringy City for Ash to come to him, which Ash eventually did in Chapter 13.
The Tauros would likely have had trouble breaking out of the Safari Zone and couldn't cross Kanto in their numbers without causing all kinds of problems, so again, it's simpler for them to wait for Ash to come to them, which he does in Chapter 19.
Considering that Ash released it while he was still in the Orange Islands, Lapras may not even know where Pallet Town is and so wouldn't have any idea where to go if it started looking for Ash. Eventually, Ash receives Lapras as a gift for helping to protect Silph. Co as a Mythology Gag to the original games; chapter 17 establishes that Lapras's herd now live in the Safari Zone as a protected species, with that specific Lapras being kept in Silph Co. as part of a program intended to try and improve their breeding potential.
Snorlax… that big lump may be loyal to Ash, but does anyone think it would even be capable of staying awake long enough to get to Pallet Town from the Orange Islands? Eventually Snorlax is found at the Route 13/14 intersection (as another Mythology Gag), but has apparently literally spent so much time asleep that it didn't even realise what had happened until that moment.
When Ash's Johto and Unova teams start their Ship-to-Ship Combat about whether Misty or Iris is the better potential match for Ash, the only ones to abstain are Heracross and Sandile. Why would those two stay out of it? Basically everyone else has reverted to their first stages and hence regressed in terms of their emotional maturity, and almost none of them 'grew up' beyond their second stage of evolution anyway; Palpitoad could be in the equivalent of a 'moody teen' phase, and Pidove/Unfeazant spent comparatively little time with Ash and so had less opportunity to grow, but Heracross was a single-stage Pokémon and Sandile was the only other Unova Pokémon to reach his final stage.
Word of God has stated that Heracross isn't that bothered because he tends to focus on sucking Bulbasaur's sap and so isn't particularly bothered about Ash's relationship status.
Porta Vista has a tunnel connecting to Gringy City, which is a known Grimer/Muk colony. Why? The ship-building industry produces an INSANE amount of waste. Who eats waste like that? The Muk line.
Falker is the only Johto Gym Leader to know anything about Ash's abilities when they receive news he's been appointed as Lance's champion; most of the others assume that Ash is just riding on his father's reputation. Word of God confirms that the original idea for his knowledge was just because he is the Gym Leader nearest to Kanto, but others have pointed out another reason; in his role as the local master of Flying Pokemon, Falkner runs the local air mail service, giving him access to more information than the average Johto citizen.
Fridge Horror
Brock mentions that Misty got off lightly compared to some other people that he knows of who had bad encounters with Shadow Pokémon. Considering that Misty's current state left her terrified of even talking to other people before Ash saved her life and reduced to a far more timid version of her familiar self, that raises questions about what other people ended up like after similar events.
At one point, Iris dislocates her wrist and resets it herself, musing that she has experienced worse kinds of pain, both self-inflicted and caused by others; just what has Iris been through where a dislocated wrist is a minor injury?
A mix of this and Fridge Brilliance; XD-001 (the Shadow Lugia) literally changed colour to match Yveltal, the original Shadow Pokemon, because it was just that corrupted by the process, with Yveltal in turn changing to its Shiny colour when it was purified.
