There could be any number of reasons. Your comments may have fallen into their spam filter (Not saying they did, but it's a possibility), they may be waiting in moderation, or the blog owner may have decided to close that particular post for comments altogether. It really depends upon who administers the blog. Comments on blogs are not a right, but a courtesy that blog owners extend to their readers.
What type of comments have you been writing? Short comments like "nice post" or "thanks for posting this" can often end up on the scrap bin. Generally, good bloggers want comments that promote discussion of their posts, much the same as those we expect from our members here. If you have a question about a certain blog comment, your best option is to contact the blog admin and ask why your comment wasn't published. Even if they don't have a posted contact address, if you use something like admin (at) blogname.tld or info (at) -- of course substituting the sign for the word 'at', your question will usually be directed to the blog admin.