Big dips on both sides of the mattress and we aren't big people!!!

After our bed was 4yrs. old it got dips on both sides of the mattress. With a HUGE lump in the middle.  We aren't big people either!! They fixed twice now and once again were having the same problem.  It's hard coming up with money to buy a purchase with this expense and then to be very disappointing and can't sleep many a nights!!!  This problem needs to be solved!!!