I had troubles with figuring out why one pair of primers worked before but stopped working suddenly. I spent the whole week testing different conditions and combinations of primers but still no progress. I ordered another pair of primers and it just worked out of the box, which makes me really doubt whether it worth to spend so much time on testing the old primers. I think it still worth but it is better to order new primers (since I have so many choices from my software tool after one design). Overall, PCR is kind of weird and a lot of time I cannot figure out why some pair of primers did not work, and the best way is just ordering new primers. Primers are just like life and you can try to change it as your way, but if you did not succeed then you have to live as its way.