I just got Invisalign and none of my trays cover my upper right back molar, is this okay?

Many times a semi impacted (stuck) wisdom tooth, the last tooth in the mouth, is covered with gum tissue and therefore is not covered by plastic aligners.  Otherwise, all teeth should be covered.  The reason for covering teeth, even if they are not in the plans to be moved by the aligners, is that if they are not covered there is a chance they may "over erupt" meaning since they are the only teeth not being kept in place by the aligner they may migrate out of the gums further to contact the apposing tooth/plastic.In the situations where a doctor decides to proceed without full coverage of all fully erupted teeth the Doctor understands that these teeth need to be monitored closely during the treatment for any super eruption.  If the Doctor told you the only reason for not covering these molars wasbecause of a discrepency in the impression then perhaps a new impression would help alleviate these concerns if the teeth can properly be picked up in the mold.

The answer to your question is as follows:The aligner must completely surround the furthest back tooth that is to be moved OR/The aligner must partially cover the furthest back tooth that is not to be moved.In order for a tooth to be moved, the aligner must touch the entire tooth to get the adequate forces and pressures on it.  If it is not moving, part of the biting surface has to be covered to prevent it from growing further out of the gums.Aligners may be short because of improper impressions, incomplete scans, or because of gum tissue.Hope that helps!