Both of my toilets have difficulty with flushing everything down. Most of the time it takes 2 or 3 flushes and sometimes 4 flushes. I have replaced the innards and still the same problem. I have low water pressure and hard water. The house was built in 2002. Should I replace the toilets or do y'all have a better suggestion?

With low pressure you may need to install a pressure bladder tank. However, make sure the rim holes are open. Look under the toilet rim to check that the holes are fully open. Clean them out with a coat hanger and also make sure the hole at the bottom of the bowl is open. Put your finger in the hole and flush, should get good flow to start the flush.