Please, try going to the Options--> Settings--> Downloads-->New download--> Advanced--> Miscellaneous and select "Ask" or "Resume" in the box "If the file already exists".
Please, right-click on any of your downloads and select 'download properties'. Proceed to the 'Connection' tab and set the maximum number of segments to one. Should it not solve your problem, simply delete that download from FDM's list (without removing the partly downloaded file) and add it again, pointing it to the same folder. When you are asked what to do with the existing file, choose 'resume'.
Please, right-click on any of your downloads and select 'download properties'. Proceed to the 'Connection' tab and set the maximum number of segments to one. Should it not solve your problem, simply delete that download from FDM's list (without removing the partly downloaded file) and add it again, pointing it to the same folder. When you are asked what to do with the existing file, choose 'resume'.