I bought a used 2000 Subaru Forester with about 149k miles on it.

Its been great! I have about 164k miles on it currently. 

Now it has been overheating when stopped at a light or idling.

When I open the hood I noticed the reserve coolant is full. 

But the radiator appears low.

Why isn't the radiator drawing coolant when its low from the reserve?

What can I do to fix it?

Thanks

we never want this to be headgaskets, but it often is. prepare yourself for that likelihood.

things to check/try;

First, make certain the radiator fans are working.

the car MUST have a working OEM-style thermostat, many aftermarkets will not work.(see pic below)

a new radiator cap (OEM or maybe Stant) would be a good idea.

Pull the o'flow tube out of the bottle and cut the end at a 45* angle.

Put a tiny clamp or 2 small zipties on the o'flow tube at the radiator neck's nipple.

 

it can be tricky filling many Subarus with coolant, best to get the nose uphill or on ramps and cycle the car from cold, to warmed up a coupla times while watching a little coolant be sucked in from the o'flow.

good job BTW actually looking in the radiator to check the level - don't trust the level in the o'flow.

 

 

 

Are the cooling fans running when it overheats?

Low coolant level will lead to overheating, but the fans should turn on if it starts to get hot.

 

Top off the coolant level, replace the radiator cap, and verify that the fans are working.

 

There should be two green plugs with a single wire each under the dash near the steering column. Connect those and turn the key On to put the ECU into test mode. It will cycle the fans on in low, then high speed and off in about 2 second intervals. Make sure both fans work.

Turn the key off and disconnect the green plugs to take the ECU out of test mode.

My guess would be head gaskets. Trying squeezing the bottom radiator hose when the engine is hot to feel for the thermostat. If its open you it will be fairly easy to squeeze. if its closed there will be some pressure. you will hear the little pin on the thermostat rattling. 

+1 on likely headgasket beginning to fail. How to check coolant level- note coolant level in the reserve tank. Sharply squeeze the upper radiator hose. Listen for the jiggle pin and air gurgles. There should be little to no air.

 

Running over normal temperature, (does not have to get to the red) while low on coolant accelerates headgasket failure.

 

If closely monitored - as in check before every cold start - I've gotten away with delaying the headgasket repair a few days to a few months by just keeping the system refilled, but it is temporary and unpredictable trick to buy time to find the original leak, or reseal an engine to swap in.

It's an internal headgasket failure. 

 

but rule out simple stuff first as already mentioned: 

1. cooling fans coming on?

2. properly bleed and refill it to make sure it's not trapped air. 

3. replace radiator cap

4. look for leaks