What more ways could the experiment have went wrong?

My experiment was about Genetic analysis with PCR, I wrote this as summary "Unlike other organisms, humans have two sets of chromosomes and two sets of genes. Therefore, the TPA-25 gene can insert with Alu in one of three ways, namely +/+, meaning that Alu has been incorporated into both chromosomes, +/-, meaning that Alu has been found on only one chromosome of TPA. -25 gene, or -/- which means Alu has been removed from both chromosomes. Where an Alu element may be present on the TPA-25 gene, primers will be applied. In the absence of Alu elements, this primer copy will cover a region of about 100 bp. In the presence of Alu elements, 400 bp will be covered. During the experiment, the aim is to determine which type of Alu is included in the two chromosomes of the TPA-25 gene. Based on the results, it appears that Alu is absent in both chromosomes of the TPA-25 gene, namely +/+ indicating that Alu is incorporated in both chromosomes of the TPA-25 gene, +/- indicating that Alu is present on only one of the TPA-25 gene chromosomes, or -/- indicating that Alu is absent from both chromosomes. A number of factors could have contributed to the incomplete result we obtained, such as not precisely pipetting the supernatant or chelax into the wells, the transfer of the supernatant to a new eppendorf tube, and the presence of chelax interfering with the PCR reaction by binding to Mg 2+ ."