Lisa,
As F. Scott Wilson mentions, payment can be delayed if the LC wording leaves anything up in the air. Keep in mind that the banks are only concerned about the accuracy of the paperwork and its conformity to the LC. They do not have the means to judge the adequacy of the material. Obviously, that can make any buyer nervous.
To eliminate any need for the buyer to approve the material on their side, there are independent surveying companies who can, for a reasonably fee, inspect the material prior to export and watch it being loaded into a sealed container. It could even be an individual the customer knows and trusts. Your goal is to remove any LC payment contingency based on acceptance.
If you can convince the buyer to issue a LC payable at sight rather than usance, you eliminate questions of material acceptability since payment is due when the original documents are presented to the bank.
After all is said and done, depending upon the closeness of the relationship between the buyer and his bank, they can delay and dispute payment on flimsy documentary discrepancies as leverage for some consideration. While it is rare and while it is difficult to withhold payment indefinitely, no transaction is completely risk-free, including an irrevocable LC.