When it comes time to put in an offer on a home, whether it's a purchase or a rental, always submit your offer in good faith.
Too many times, others do not negotiate in good faith, return calls or emails as promptly as you or your agent do, and may have hidden agendas as to why they are hedging on accepting your offer, or even counter-offering your bid, and to top it all off, you may not ever find out what their reasons are.
Recently, I had a client who placed an offer in on a property where it appeared that the listing agent and owner did not negotiate in good faith, and as upsetting as that may be, it can happen. It's very unfortunate, but you must remember that not everyone operates with the same high principles that you do.
Certainly more often than such is the case, availability is misrepresented, and/or the presence of offers that are "on the table" is communicated to you or your agent in an effort to get you to adjust your price or terms. Sometimes the homeowner "hedges" or bides their time with your offer, and there is no way of telling what is truly accurate and what is not. Sellers can also change their mind about selling or renting at the drop of a hat. You just have to go by your gut feeling. If you or your agent sense this type waffling is happening, or that there are no other offers that have been, or just been submitted, proceed with caution, and always have a "Plan B".