I think the 306 is a good machine. I have a couple. You need to have the manual for it.
It does have a couple of gotchas so the manual is a must. It takes a special needle
not the standard 15X1. It takes a 206X13 which you can get on Ebay. It also takes
a special bobbin which you can't buy in stores. You can also get them on Ebay but
try and find used originals.
You can find Chinese replacements but they aren't great quality and sometimes don't
work well on the bobbin winder.
Your problem looks like you don't have the 206/306 bobbins. 15 class bobbins won't work.
There also are two different bobbin cases. One with a cutout by the needle and one without.
You may have to buy one of each to find out which one fits your machine.
If you use the wrong needle or bobbin case the needle can actually hit the bobbin case
while sewing.
The main complaint with the 306 class machines is that it is a little inconvenient to
remove and install the bobbin case. Otherwise they are a heavy duty machine and
sew great.