Have a one storey home on concrete slab foundation. Heating is forced air natural gas furnace(believe it is a Lennox) with the ducts in the slab foundation. Home is apprx 50 years old. Have only owned the house for 10 months. All of the ducts have good air flow but two of the vents only blow cold air and the others blow nice warm air when the heat is on. There was no apparent collapse or damage to the ducts that we could see when we had them scoped at purchase time. Any idea why the two vents only blow cold air(luke warm at best if the furnace runs a long time) or what can be done to fix or resolve the issue?

Are they the furthest vents from the furnace?
My house is the same age, but not on a slab. It is 45' long. The furnace is in one corner of the basement. The vents emit progressively colder air.
Because 2 walls have been removed at the cold end, I cut 3 ducts off and sealed them. Helped a lot. Warm air drifts into those areas. Made more sense than cold air blasting out.
I assumed in my case that it's a simple matter of losing heat across the cellar, despite all of the ductwork being wrapped.

I see on This Old House that more modern installations use smaller ducts and more fans, designed to run more often but use less energy.

If you are in the northern US, and moved into your home last May-June, then this was your first winter. The ground (and slab) take months past the start of cold weather to get the coldest they are going to get, because heat from deeper down is still working it's way up. April could easily be the coldest month. I found stats on this once years ago, but don't remember exactly what the below ground 'heat lag' chart states.

How many vents do you have, and where are these vents located relative to your heat source? What is the square footage of your home? What kind of heat do you have and what climate/region are you in?

House is just under 1000sq ft, furnace is located in the centre of the house and the 2 bedroom vents that are cold are on the east end of the house. The living room which is on the west end (about the same distance from the heat source but opposite direction as the cold bedrooms) but blows warm there. There are 8 vents, one for each bedroom(3), 1 bathroom, two each in the living room and kitchen. Furnace is only about 5 years old and is a forced air natural gas.

Shut most of your living room vents and see what you got.

Question that might be too obvious: bedrooms are often on a separate zone(s). Is that possible?