system
Given the following post, respond as if you're commenting on Reddit:
human
[Stargate] So, in SG-1, in season 1 the Jaffa armor seems to be pretty tough. Six jaffa or so are able to abduct a soldier and shrug off rounds. In every other season, they are extremely weak. Why?

The way I've internalized it is basically 1 or 2 things, or both.

1. The armored Jaffa in the beginning of the first season are Serpent Guard, so they'd probably have tougher armor,. All his other plebs can die en masse for all he cares, but he needs pretty strong armor on the people who actually watch him, and
2. The P-90, which isn't used in that scene but is the main weapon used by SG-1 later, was specifically designed as an armor-piercing PDW for non-combat troops during the Cold War. It was one of many created so that, if a war ever kicked off, troops that do things like logistics or other things far from the front lines would be able to defend themselves against Soviet paratroopers, who would likely wear body armor and use light vehicles. Ergp, we could assume that impacted it.

Plus, every other weapon could just be using armor-piercing rounds, whereas the troops in the first scene were in the middle of a mostly unused facility in the middle of the U.S. - They probably weren't issued armor-piercing rounds, and .223/5.56 is notably a small round anyway.
gpt
The Jaffa used in season 1 were apophis' best troops. He was absolutely not taking any chances with the world that no one knew about that suddenly showed up thousands of years after vanishing and killed the most powerful system lord with a small team of soldiers. His own forces would have needed a massive army and likely would have failed accomplishing a similar feat. 

The soldiers guarding the decommissioned stargate facility were packing weaker rifles and ammunition that are fine for guard duty but worthless against armored targets. In later episodes and seasons they switched to more powerful rounds and specialty ammo meant to punch through armored targets at close range as a counter to Jaffa tactics. 

Later Jaffa encountered are weakened by years of civil wars and goauld overconfidence. They're still deadly threats but rarely wear full armor outside of ceremonial positions. Many of these Jaffa have armor and tactics focused on fighting other Jaffa and energy based weapons. The projectile weapons of earth are able to break through the armor at weak points and more accurately hit the unarmored body parts that staff weapons and zats are unable to accurately hit. 

And finally there's the possibility that apophis allowed his Jaffa to use personal shields for their early missions. He felt the potential risk of earth was worth giving access to technology normally reserved for goauld exclusively. He later revokes this access to prevent traitor jaffa or the sg soldiers from acquiring the technology for themselves. Given the extremely territorial and secretive nature of the Goauld his shield technology wasn't known to other system lords until later after his deaths and defeats and acquired by rival lords unable to mass produce the devices for their jaffa.