The neo-grammarians claimed that sound change had to be regular. While this certainly appears to be true in the majority of cases, it doesn't seem to be true all the time. For example, I produce a schwa in the second syllable of "England", "Finland", "Greenland" and "Iceland", but not in "Thailand".
Why is this? Some people probably do produce a schwa in "Thailand" but I don't. I suspect that certain pronunciations are due to what we're exposed to at an early age. I probably heard the pronunciation with a low front unrounded vowel in the second syllable and thus acquired it.
Also interesting is that I pronounce "Slav" with a low front vowel like the majority of Canadians but have a low back vowel in the word "Yugoslavia". In the word "Yugoslavia" I suspect that Canadians use the low front vowel less than in the word "Slav". Though the word "Slav" is in the word "Yugoslavia" the syllable structure is different. In "Slav" it's followed by a syllable-final consonant whereas in "Yugoslavia" it's in syllable-final position because the syllable structure is Yu-go-sla-vi-a. The low front vowel never occurs word-finally so this be the reason that the syllable-final "a" in "Yugoslavia" is pronounced by many Canadians as a back vowel.
Also, I've noticed that I usually don't pronounced the first "e" in "interesting" but do so when I say it emphatically- in other words, when I say it slowly and in isolation. In such cases, I tend to pronounce it. However, in the word "whitener", I definitely do pronounce the "e" but it's possible that this is really a syllabic nasal which shares the same place of articulation as the preceding plosive.
Unfortunately for the neo-grammarians, it appears that sound change isn't always 100% regular. The reality is that language simply doesn't function exactly the same way as math and physics where the rules are absolute. For this reason, I view language as a blend of art and science and not simply science.