Tech upgrades not raising rarity chance

As the mark level and rarity of the item you are trying to upgrade increases, the % chance of rarity decreases and can drop below 1%. The chance for rarity upgrade only displays whole percentages which means no decimal places are displayed.

The % chance of rarity increase can fall below 1% which means you may need to use many superior upgrade kits before you see the % chance increase by 1%. For example, I think if you are attempting to upgrade ultra rare Mk XIV to epic Mk XIV you will need to use 4 superior upgrade kits and 4 1.5x quality accelerators to increase the chance by 1%. Or use 6 superior upgrade kits to get the same result.

That's how the mechanics have been set up. If you want epic, then you gotta work for it.

However, very rare Mk XIV is typically fine unless you are overly obsessive with DPS in the game. The biggest boost for weapon damage is when you go from Mk XIII to Mk XIV regardless of the rarity. The best advice I can give you is:

1 - Wait for Double Tech Point weekends to do most of your upgrades. It has no effect on the % chance to upgrade, but in order to be able to click the upgrade button, you will need to fill the Tech Point prerequisite.

2 - During the Anniversary Event farm the Omega Particles to craft Omega Upgrade kits which has 4x the tech points and research bonus of Superior Upgrade Kits. However, most people use Omega Upgrade Kits on Mk II to get as many chances to upgrade an item to ultra rare or epic.

Well, like I stated, for weapons the largest increase in DPS comes from upgrading from Mk XIII to MK XIV. Therefore, very rare Mk XIV weapons will be fine

You can always buy refined dil. The exchange rate is 300:1 so for the cost of a cup of McDonald's coffee (which isn't bad for the price; though I prefer Starbucks) you can buy 30k refined dil.