[BioC] NA values in Biomart query

[Chapeaublanc Elodie]

Hi, 
I want to retrieve some annotation informations from "ensembl exon id" by using a biomart query : "ensembl gene id", "ensembl transcript id", ...
After my "getBM" query, without error message, my object "results" contains lot of "NA" values but if I re-send a query on "NA" values id, I successfully retrieve informations.

Example : 
library(biomaRt)
ensembl <-  useMart(host="jan2013.archive.ensembl.org", biomart="ENSEMBL_MART_ENSEMBL", dataset = "hsapiens_gene_ensembl")

results <- getBM(attributes=c("ensembl_exon_id","ensembl_gene_id"),filters="ensembl_exon_id",values=ENSE,mart=ensembl, uniqueRows=TRUE)
My object ENSE contain 512120 exon_id.

Why I obtain lot of "NA" values when my value option  is large ? Must I to split it ? and Why ? 

Thanks, 

Elodie

sessionInfo()
R version 3.1.1 (2014-07-10)
Platform: x86_64-pc-linux-gnu (64-bit)

locale:
 [1] LC_CTYPE=fr_FR.UTF-8       LC_NUMERIC=C              
 [3] LC_TIME=fr_FR.UTF-8        LC_COLLATE=fr_FR.UTF-8    
 [5] LC_MONETARY=fr_FR.UTF-8    LC_MESSAGES=fr_FR.UTF-8   
 [7] LC_PAPER=fr_FR.UTF-8       LC_NAME=C                 
 [9] LC_ADDRESS=C               LC_TELEPHONE=C            
[11] LC_MEASUREMENT=fr_FR.UTF-8 LC_IDENTIFICATION=C       

attached base packages:
[1] stats     graphics  grDevices utils     datasets  methods   base     

other attached packages:
[1] biomaRt_2.20.0

loaded via a namespace (and not attached):
 [1] AnnotationDbi_1.26.0 Biobase_2.24.0       BiocGenerics_0.10.0 
 [4] DBI_0.2-7            GenomeInfoDb_1.0.2   IRanges_1.22.9      
 [7] parallel_3.1.1       RCurl_1.95-4.1       RSQLite_0.11.4      
[10] stats4_3.1.1         tools_3.1.1          XML_3.98-1.1  




Elodie Chapeaublanc
IE Bioinformatique
Équipe Oncologie Moléculaire
Institut Curie - UMR 144 - CNRS
26 rue d'Ulm - 75248 Paris Cedex 05
Tel: +33 1 56 24 63 57
Email: elodie.chapeaublanc at curie.fr