Period & ovulation correlation

Usually my periods are exactly 28 days, ovulating on the 15th day. Last month it stretched to 30 days but my peak (LH surge) was on the 13th-14th and month before it was on the 16tj day. Now, I am so confused bc I thought I had my dates down & Ttc now & it's thrown off??? Why is 
ovulation date not getting pushed if the period is delayed???