Adding attic insulation will help. You should have at least R-30 insulation. The more the better.

Power attic exhaust fans are a hotly (no pun) debated topic. If the attic has inadequte ventalation, it is possibe for the fan to create negative pressure. This will cause cool air in the house to be drawn up to the attic, causing the AC to run more than it should. In addition the fan consumes power, so it subtracts for the savings.

I have a attic fan in my 2-story colonial. I think it is an overall positive affect. I do have a ridge vent and soffits vents. Check to see you have soffit vents and they are not blocked.

I live in a ranch house and there is currently very poor insulation in the attic. I've been told that although our a/c is sufficient, we are losing cool air thru the attic.
Will adding more insulation make a difference?

I think that your cool air loss thru attic is more of a hot attic air infiltration & lack of sufficient insulation.
adding insulation may help, but first seal any leaks between living space & attic. once you insulate you won't ba able to access these areas from in the attic.
blower door tests will help to determine where these leaks are.
you can also seal ductwork with mastics not duct tapes.
most homes have a lot of duct leakage.
I find the air sealing and duct sealing makes a huge difference. Recessed lights are a big leakage and allow insulation and hot attic air into the house.

Will replacing the attic fan (which is very onld) help or will it end up pulling more cool air from the rooms ( many of which have recessed lights)? Also, what is the function of a roof fan?

attic fans pull outside air in through open windows. if you live in a high humdity area this is not a good idea.
if you have allergies...not a good idea. we don't do this in my area (south La.) but I read of others in milder climates who do. so this would be climate specific and dependent upon your needs.
roof vent fans...pull conditioned air from house via leakage sites and duct leakage.
what type of attic ventilation do you have?

The a/c runs almost constantly, but the rooms just don't seem to cool. And, it's a relatively small space 3 bedrooms and 2 baths.

it may need servicing, new filter, coil cleaned, more freon...lots of things
if the unit is in good shape, it may be that the house just has too much heat gain.

is this a new to you house or have you been living in it for a while and this is a new situation?
What is your location??

best of luck.

thanks for your replies.
We have been living in the house for about 15 years. The bedroom area had a new condenser/air handler and duct work put in maybe 13-14 yrs ago.
We also added the recessed lights, which I can tell are a big part of the problem. We live in northern New Jersey. Our house is a ranch with a a low-ceiling attic and the gets very little shade.
I've had the compressor checked and was told that it is operating the best it could, considering its age. Though, if you put your hand up toward the vents, cool air is blowing out.
Apparenlty, the ductwork is not good - it's been called "creative at best". Just to replace the ductwork, I've had a quote of $3400, which seems might high. We can no longer use freon here. So, I can try new ductwork to see if that helps, or have to replace the compressor and air handler and have new lines run. We did that with the other zone in our house last summer.

recessed lights allow a lot of heat into the house.
this would add to the heat gain.
creative at best ductwork..thats a new one for me..
although I've seen a few whacked ductsystems in my life!
hvac company will charge about $250 to 300 per duct
and if replacing plenums add another $200 per.
if the company has a duct design rather than just getting
in and mix matching ducts I'd want to see it up front.
ducts are sized to rooms size and cooling/heating requirements..it isn't just running any size.
and if the ducts aren't sealed..you are just pumping
air you pay to condition into the attic.
I don't understand why you can't use freon.
it may be system..may be ducts..may be a combo
of many things.
you can retro fit your recessed lights to air tight
with an airtight insert, get brand and model number
go to mfg and search for ICAT insulation contact air tight.

best of luck