I run the cuffdiff for comparing two RNAseq samples as follows:

/usr/local/cufflinks/bin/cuffdiff /ucsc/dm3/genes.gtf /data/RNAseq/Cufflink/Wide-type3/acceptedhits.bam /data/mutant/accepted_hits.bam

but in the output FPKM tracking files (cds.fpkmtracking, genes.fpkmtracking,isoforms.fpkmtracking, tssgroups.fpkm_tracking) , there are no length and coverage value (column 8 and 9 are "-"). Is there something wrong or I omitted some parameters?