I'm not native to and not good at English, neither live in English-speaking countries, so I do not know the exact reality of the diversity of English pronunciation. But I'd like to hawk some ideas from books here.
Successive two or three explosive consonants(stops) will make it awkward to enunciate each of them one after another, especially in fast speech. For English, people usually weaken or drop one or two stops in continuous speech, e.g. mortgage, looked, as long as it does not make difficulty or ambituity for understanding.
As to words like asked, looked, the 'k' sound and 't' sound are respectively back and front stops, which makes it particularly difficult to pronounce them consecutively, and so there must be one to be left out. As the final 't' sound does matter: it indicates the meaning of past tense or past particle, therefor people weaken or drop the former stop 'k' sound. As classical and graceful pronunciation, you do not totally omit the 'k' sound, instead you make the articulation and keep it for a short time and then pronounce the next sound. But in causal and fast speech, the 'k' sound may be completely dropped.
In everyday English, there would be even more sounds or sound groups be elided, which often causes difficulties for English learners to make out the words and to comprehend the phrases and sentences.