I have an old house with a brand new septic system... from septic tank through drain field. My toilet does not flush properly. It starts to flush, but stops about half way through. There is also a gurgling noise coming up through the shower drain right next to the toilet. I thought it might be clogged vent... but I can't find a vent anywhere. I opened the big line downstairs last evening and snaked it out for 25 feet... no clogs. I'm kind of at a loss as to what to do. Please help!

The Lavatory Vent should vent the entire bathroom group. You were correct in assuming it sounds like a vent problem. Locate the lav vent, (hint: It might be revented back in the attic) and snake it out. Let me know how you make out. Good luck, Tom