Everything said so far is good advice. I had similar problems when I strated a few months ago. Here are a few things I've learned:
I'd add that poking it with tongs is definitely a bad idea. If you don't have a problem with spots now, you will likely have a problem when/if you tone the prints. Just slide the prints under your 2-3 liters of developer and rock the tray and the developer will reach the paper within a few seconds.....if you are using enough developer in a larger enough tray.
Also, I found that vigorous agitation tended to build density at the edges so I like to rock the tray more gently, but continuously.
Lower dilutions are generally more controllable, but take longer and exhaust quicker.
When you notice the development taking way longer or there is a sudden change in your prints, change or replenish the developer.
Look the gallery for good lith prints. Thomas Bertilsson's prints are definintely worth looking at.
Last, get Tim Rudman's books if you don't aleady have them.
Oh, yeah.....and it is the most fun, creative and interesting printing I've ever done!