The host server staff says they don't see any log errors.or
I can update using FastFTP, with no problems. The problem must be with Blogger!These are common attitudes, which probably should be expected. Common or expected, they may not get your problem solved.
The clues are in the server logs. What the host server support personnel see as "no log errors" may need to be interpreted properly. You pay them for service, so they work for you.
FastFTP, and similar desktop clients, require a series of server profiles which you setup, by hand - one for each server. You have no problem with FastFTP because you configure FastFTP, specifically, to work with your host server. Blogger code, on the other hand, has to support hundreds or thousands of different host servers, never knowing if any 2 should be treated the same, or differently. Many host servers are specified as domain URLs. How can the Blogger code identify and track each individual domain / server URL, and know what precise tweaks are required for each?
If you look in the forums and see a few questions about FTP publishing problems already asked, this possibly indicates a problem that Blogger has caused. Find a common question with similar symptoms being reported, and add yourself to that question. When there are others sharing your symptoms, adding your symptoms to the question may help Blogger solve your problem faster.
On the other hand, if you don't see any similar questions, it may be up to you to deal with your problem. And you should start by learning to assert yourself, when you need help with your FTP publishing problem, by patiently and persistently involving the host server support staff. Ask a question in the forum - somebody has to be the first, after all. But concentrate your efforts on the host server support.
Occasionally, we see problems that are actually caused by host server changes. Keep an open mind here.