1. Pull the
   hub feeding the line and check for a broken cable or worn fitting parts
   (rounded vs square).
2. Check
   the clutch and So-On if needed.

I pulled the engine piece from the line and checked. I couldn't see anything worn or broken cable (because it's obviously still spinning at the end, just weak). But there is one thing I don't understand. At the end of the line, the is the female square fitting to the hub engine end. BUT at the part where the hub connecting to this line, there is nothing to "stick" into that square fitting. It's just a "batman" looking shape to fit into the box. No male piece sticking out. I am not sure if that is how it supposed to look. But one thing I didn't mention in original post: I did pull out the two pieces due to not using for 2 years to oil (I couldn't pull the cord initially). When I put them back in, it was like that. So it could be my fault.

I don't have Ryobi. Assuming the motor is running correctly, it sounds like the centrifugal clutch is stuck. This would be on the engine at the point where the cable that goes to the head connects to the engine. You will need a parts manual to see how it is put together.

1. Please show Pics of the cable’s ends and where
   they insert into the clutch and string’s head.
2. Does the clutch look complete assembles w/o
   excessive wear?
3. You are loosing your direct drive IMO and usually
   it’s easy to see. Power w/o
   response!!
4. Did you manually turn the cable against the
   string’s head and clutch separately?
5. Please share some good internal Pics of both end’s
   fits.

Good Luck!

I saw some youtubes at the
address below but I did not see one IMO related to your problem. Some of these mechanics “Say they welcome
questions, please see if one possibly understands your trimmer’s problem. Either way you find the problem, please share
the problem and fix. Thanks!

https://www.youtube.com/watch?v=uq4bmYi0rEk

I have one with multiple tools. The fitting from the power head end is straight forward and its square female fitting is easy to see. However the end that connects to the actual tool business part is less obvious. I have a blower that was slipping and I found that there was an outer connector that could be loosened to allow the shaft to extend into the blower fan just a little more. I suspect something similar for you. My weed whacker attachments are in a jumble now (I have a stand alone weedwhacker). I may get around to digging one out and looking at it to see if there is an adjustment.

@chas045 and @loger_gw I have taken it apart and showed 3 photos here. I turned the circle piece at the line (with the female square) and the string head is turning fine. The weird thing is that the engine is connected with this rod by 4 bolts. So internally, they just kinda pushing against each other tight to create the turning drag or something? That doesn't seem right.

1. What
   appear as clutch shoes appear to be worn in the 1st home design
   pic. Do you have a Small Engine
   Parts & Service Shop close to get their opinion? I see signs of heat. Do you see signs of wear on clutch’s
   shoes and hub (from slipping)? If
   this is the case there is probably a wear limit that will cause the
   slipping. There could be a spec on
   too much wear listed or compare with the looks of a new shoe.
2. Does
   the engine hold good high RPM when the string head slows?
3. Oops! I missed a pic and out of time for now.

Good Pics. I tried to
add light to them, trying to see wear.

@loger_gw please educate me as I cannot seem to see where the "wear" is. Here is a better pic. Is that the "round button" in the middle? Is it supposed to be square or something? It chipped a bit though but can't be that big of an impact right? Yes this engine runs and there is lot of heat. Not sure why.

Please expand your very good pic
to show the round hub the shoes fit inside and shoes also. That is where I feel I see signs of heat and wear.
The shoes are slung out from the engine’s
RPM in the hub. The hub has a square
recess for the drive cable to drive the head with the cutting line.

Someone! Please
help me out as to what I am not saying.
If you have no mechanical experience which is no issue, take it to a
repair shop for opinions. You have
mechanical experience from what I see but maybe not this process.....

What are good safe mechanical test for the cause of lack of
power if it is not easily seen? Such as, at the link below, is there a safe way to see or test the clutch for Good Torque
and RPM as the string’s spool slows?

What parts on the diagram or another diagram are possibly
bad? I’m running out of suggestions w/o
the trimmer in my hands.

http://www.ereplacementparts.com/ryobi-ry29550-string-trimmer-parts-c-7931_15633_15647.html