Why is it sometimes much easier to see your veins? does it mean that something is wrong?

No: some people have prominent veins as compared to others Also based on local pressure above the vein area and temperature changes may make veins more prominent at times

Fluid levels: It may mean that your intravascular fluid levels are higher. It could be an issue if you have heart failure. If you have healthy heart. It is probably normal.

Normal or Blocked: Most normal veins are hidden deep inside, but some are near the skin surface, show more if you are thin, hot, exercising; more if hydrated, less if dry. Normal veins enlarge when hands hang down, flatten when up. Abnormalities increase size of veins. Obstruction (clots, cirrhosis of liver, pregnancy), pressure (heart failure, etc), & prolonged standing make veins stand out. Seek doctor's advice

Visible vasculature: No. It has to do with how superficial some veins are and how well hydrated you are as well as how easily the veins are emptying into larger veins. This is all normal anatomy.