The same thing has happened to me on occasion... and it's almost entirely due to the cold temperature.

The hatch struts aren't pure hydraulic... they're gas-filled as well.

Within a constant volume, gas pressure rises and falls as a function of temperature.
So when it's cold out, the gas pressure is lower and the overall lifting capacity is reduced.

If the hatch struts worked fine during the warmer months, I'd just use a 2x4 for a prop until the spring thaw. It's not particularly elegant, but it's cheap.

h: