Firstly what do you think is blocking it ? we get drains blocked at work daily(hourly) and the biggest culprit is wipes !!! these things cause mega trouble, as they don't breakdown. For the drain to work well it needs to be the correct diameter 4" and have the correct fall"  I cannot remember the grade off hand, but this is fairly critical, as too level and the solids just stay put, too much and the liquid flows over the solids I have a great  PDF on making your own septic system out of 2x 55 gallons plastic drums which explains everything really well but of cause I cannot find it at the moment.
Tree roots are another real problem in drains they catch everything ! are there any bends in the drain that could be causing a problem ?  without a doubt drain rods are the best way of clearing a blockage, but even these can fail to move some blockages, and the only other method is jetting.
Hope you sort it out,as there aint much worse than blocked drains especially in warm weather LOL!!!!!!