If the battery/computer…

• Is more that 2 years old.

Batteries are very tricky. Because you have to think of them as fuel tanks. Batteries have a capacity, and when you charge them you're refilling the tank. Although, over time after several drains and refueling. The actual capacity gets smaller and smaller. So it's charging to 100% of the very little to no capacity.

• Has been plugged in constantly when used.

As weird as it sounds this is actually really bad for the battery. Yes, it does keep it charged constantly but batteries do have a “memory" to say the least. If a battery doesn't get the chance to deplete and refill, its at a constant state of depleting. Rather than refilling.

• Was left unused for a good amount of time (more than 2 or so months)

What happens is the battery voltage goes below the amount needed to power the device on its own and applying the power charging source is barely putting it past that threshold for it to work as it's plugged in but not unplugged.

• Battery is not the original and was replaced with a “cheap” replacement.

This goes hand in hand with the previous one. When batteries are made they're sent to a warehouse until sold. So they're “New" because they haven't been used but they're not charged while waiting to be sold and discharging.

Overall a new battery will fix the issue. My recommendation go to a Battery Store such as Batteries Plus Bulbs. They'll get there batteries fresh each month and maintain them as well as they'll come with warranties. You'll pay more but it's cheaper than a new laptop.

If these do not fit the criteria of your issues then I would recommend contacting the manufacturer.