After a moderate rain,toilet won't flush,however ,shower/tub will drain even if it runs long time.On septic system,had drain field replaced 14 yrs ago,only 2 people live here,got washing machine draining out diverted from septic tank.As soon as rain stops,sun comes out,all works again.Also,even in complete flood of rain,no smelly yard,no puddles,nothing.No signs of worn out drain field as I had when I had it replaced.It was 50 yrs old then.I can't for the life of me figure this out.Asked a friend who owned a plumbing company,been in business 55 yrs,said he has no idea,doesn't make any sense to him either.Please help,disabled,have no money,thought I took care of this when I had drain field replaced.Note,had a plumber cut out cast iron pipe and replace with PVC several yrs ago,in middle of 5 yr drought.When drought broke and rain returned is when it started.

The drain field does not have to be "worn out" to become flooded while it is raining. And, if it floods it can cause just the toilet to not flush even though everything else works properly. It would be a function of how the toilet pipe was installed, and if so, there is no easy or inexpensive fix.

You may have to divert the rain water away from the drain field, ditches, piping, gravel, or a combination of the above can be used.

Septic systems are both private and different. Without being there is it most difficult. Your issue requires an onsite inspection to verify that every segment of your plumbing is working as designed. I'm guessing a partition in your wasteline that is blocking solids.

Best Wishes

You did not suggest an umbrella.