Cost of giving incentives is higher, and the respondents are also not getting bothered by the incentives. i.e. the opportunity cost I am willing to pay is not fetching optimum results.
You ask a good question; in this day and age when people are inundated with electronic communications, much of which is "spam" (or worse), how do you entice recipients of solicitations to give of their time in order to furnish someone else with data for their study--especially without incentives?
There is evidence that people will respond more favorably to a plea from a student who has to complete the project in order to complete their degree. The only caveat is, in most or all of those instances that I recall, the persons solicited were in an academic institution (so they may be more receptive or more sympathetic to the plea).
If the solicitation is clearly endorsed by someone respected by the target pool of respondents, this, too, can help boost participation rate.
Making the solicitation more personal can also help. That is, a personally addressed, first-class (or overnight delivery) letter generally gets more attention than an email solicitation from someone unknown to the potential participant.
If the apparent payoff of the research is judged important by the target pool of respondents, this can help in participation rate. So, make clear what and who might benefit from the results of your study.
Thank you @David Morse. You always actively respond to my queries.
Yes, I am doing the same i.e. personally meeting the respondents and explaining the purpose of research. Respondents actively participated in the first stage by listening to my pleas and purpose of research i.e. very near to their work matters. But for the second stage their response and attitude is very critical due to some contextual factors like indifferent attitude of general public towards the worth of social research projects especially longitudinal research designs.
But, as a student I am still trying, and hopeful to attain the desired sample size with fruitful results for my thesis.