Could you please let me know what I miss in this code ? It couldn't retrieve the symbol for ENSG00000275038, while the web interface returned AC091980.2. It works for most of genes but returns NA for a few genes.  Thanks for your help!

ensembl <- useMart("ensembl",dataset="hsapiens_gene_ensembl")  
getBM(attributes=c('ensembl_gene_id','hgnc_symbol'),
        filters = 'ensembl_gene_id', values = "ENSG00000275038", mart = ensembl)

  ensembl_gene_id hgnc_symbol
1 ENSG00000275038          NA

The "AC091980.2" is the gene name, not its symbol.   To retrieve the name, here is the updated code:

getBM(attributes=c('ensembl_gene_id','hgnc_symbol','external_gene_name'),
      filters = 'ensembl_gene_id', values = "ENSG00000275038", mart = ensembl)

Note the addition of the "external_gene_name" attribute.

For your particular entry, there does not seem to be a gene symbol. If you look at another entry, for example - http://useast.ensembl.org/Homo_sapiens/Gene/Summary?db=core;g=ENSG00000139515;r=13:27920020-27926231;t=ENST00000381033, you'll see under the summary:

"PDX1 (HGNC Symbol)"

In your entry, you'll see:

"AC091980.2 (Clone-based (Ensembl) gene)"

(edited)