So many things can go wrong. Mostly, though, it is the singers who fall back into former ways and do not think of the recent "fixes" the director asked for. This occurs especially when a piece has been in the repertoire for some time or the singers learned it previously under different conductors. A wise director will allow enough rehearsal time to revisit the tricky spots several different times before performance. This is called "transferring." The director takes time to check if the performers have transferred the new techniques, skills, interpretation, timing, et al, to reach performance level. Call it a test, if you will, but it must be done. The singers need to have enough practice to let the changes seep down deep into the fabric of their being.
Muscles have memory. Singers sometimes fight against previous muscle memory in order to put in new or different technique or interpretation. It is not easy to rebuild the muscle memory, but once achieved, it remains for a long time. The trick is to put the correct techniques into the muscles from the very beginning. The most common problems are vowel placement issues, proper support of the tone throughout the varied vocal ranges, and breath management.
Getting many voices to sing with unified vowels can take the majority of the rehearsal time. Practically every conductor has his own preferences in the pronunciation of English vowels. Regional dialects win out in most cases. Many conductors tend to prefer the high British choral tradition and train their singers with those sounds in mind. The main object is to get the singers to all agree on the same vowel formation and placement. That is how they will tune successfully.
Harnessing and taming the power of individual voices can be a full time job for a conductor. Some voices are big and loud and have a timbre that comes out of the fabric of the rest of the group. These voices may have wide vibrato or possibly a reedy or brassy quality. What is considered desirable in a solo voice, is not necessarily the best for a choral group. Convincing these singers to control their sound and not hurt their delicate artist's egos can be a daunting task. A conductor will want to have good musicians and trained voices in his group, but the singers must learn how to manage their sound quality to blend in with the group. They must learn to support their tone quality a little differently in order to blend. The vibrato must be tamed by singing with more breath. The singer must become aware of how to use the light head voice even in the low registers. Those singers will not be happy singing with such control, so the conductor should remember to choose pieces occasionally that will allow those with big voices to let loose and sing with their natural power.
Part of breath management is preparing for that first note. This can be the most problematic of all the issues the conductor has to deal with. Getting everyone to take the breath, and time their entrance to come in exactly together. Beginning with a consonant helps, but many times the first syllable will begin with a vowel. The conductor may spend a lot of time working with the singers to get them to take their breath together on the preparation beat, in the shape of the vowel, and come in exactly on cue. This is tough and takes a lot of practice. It takes the same effort to get the singers to come in together between phrases. Often, the singers relax the rhythm when taking a breath and are late coming in on time in the next phrase. Proper breath management is the key and timing the breaths to fit the rhythm.
The good conductor will always be checking to see if what his group rehearses actually transfers to performance!