kinda hard to answer this with seeing it or photos
i have a septic tank but i've upgraded during the years
what i have done is install a pumping chamber directly from the outlet of the septic tank, this pumps to another septic tank which is at ground lever eg it sits on the ground, i've filled it with soakage stone and a layer of wicken peat,
then a outlet to a manhole which then is percolated
sounds to me your problem may be 2 things, your ground is prob clay which holds moisture also your water level may be high thus it wont percolate during heavy rains/wet periods etc
2nd
which is related to the above , ground water is finding its way back into your percolation ducting then into your tank, you need to check the heights of the area near your tank/percolation duct's , also sounds like your ground is saturated
re what i've done it didnt cost a lot as i engineered it myself and carried out the works app figure was about 800e but i've never had a issue since with it
as i said really hard to tell without seeing