Considering that 2-stroke engines are pretty good now-a-days, I would argue that the trimmer head is the most important functional part of a line trimmer.
And when you look around at the variety available from the major line trimmer manufacturers you start to appreciate why the heads fitted to Ryobi's entry
level models do not function properly... i.e. they were designed with only one criteria in mind: cost of manufacture!

However there are some very good trimmer heads available that ARE designed to do the job, e.g. from Stihl et. al.
But if you don't what to go out and but a Stihl you can get an aftermarket head for your Ryobi.

When looking for the leader* in 'design', the number of patents taken out is generally a good indication and, going by the number of patents taken out by Ryobi,
they are not up there with the leaders.
*A Mr. Richard A. Proulx, of "Proulx Manufacturing Inc." seems to be a leader in this field. Check out his latest patent application here:
http://appft1.uspto.gov/netacgi/nph-Parser?Sect1=PTO2&Sect2=HITOFF&p=1 &u=/netahtml/PTO/search-bool.html&r=1&f=G&l=50&co1=AND&d=PG01&s1 =Proulx.IN.&s2=%22Richard+A%22.IN.&OS=IN/Proulx+AND+IN/%22Richard+A%22 &RS=IN/Proulx+AND+IN/%22Richard+A%22

I ended up buying a good aftermarket head for AU$50, and it's made in Australia to boot.
Since then I have only felt the need to curse the Ryobi when the line runs out or when it goes out of tune**.

**Another problem I had to fix on the Ryobi was a sticky throttle. Initially I thought it need a tune up because I didn't have the throttle control I expected
(the engine was running at nearly full speed most of the time). While attempting the tune-up I discovered that it was actually the throttle trigger that was sticking...
After removing the covers I discovered that there was no clearance between the plastic trigger (where the throttle cable connects to it) and the main shaft of the trimmer.
Grinding away a mm or so off the shaft with my Dremell did the trick and I managed to tune it quite niceley as well :)

Lampet

It almost sounds like you have your line wound backwards. Think about it - it goes back into the head and you can not use the bump head.

Just try reversing the direction of how you wind it.

With the Ryobi (single) spool, I always made sure to wind on the (paired) lines in the direction of the arrow that was moulded into the plastic and always kept the total length of line less that 2 metres (i.e. 1m for each side) in an attempt keep the wound spool as tidy as possible. In operation however, tangles inevitably occurred on the spool.

In any case, it always seemed to me to be very optimistic to expect two lines - wound on as a pair onto a single spool - to feed out smoothly through the opposing exit ports of the head without ever crossing over each other and creating a tangle.

If you have a split spool in the head of 'your' Ryobi and you are winding the line on in right direction (as marked?), then it should feed the line out at least as good as my aftermaket split spool type.

Lampet

The bump knob screws into the spool assembly with a reverse thread screw. The screw is held into the bump knob with a washer that seats into the plastic inside the knob. There is a bit of slop because of this, which in my case meant that the bump knob screwed down too tight onto the assembly. And that prevented the spool from moving inside the casing.

Solution: Unscrew the bump knob and add an extra washer to the bolt (one should do it, but you may need two) Works like a charm now.

Also my 2 cents on winding the line: make sure the line is wound on the spool fairly tightly, and try to make sure the pair of lines stay together. That should help prevent the line from getting bound up in the spool.