featureCounts outputs zero raw read counts!!

Hello all, I have a set of lncRNAs identified from a plant species for which I need to perform differential gene expression through DESeq2 package. So when I run featureCounts of "Rsubread" package of R to generate raw read counts of the lncRNAs, the output counts.txt shows read counts "0" for most of the lncRNAs, which shouldn't be the case as I have inputted only those lncRNAs that have FPKM value of greater than 1 and hence are expressed in the samples. My featureCounts command line was :

    files=c("Con-1-sorted.sam","Con-2-sorted.sam","Con-3-sorted.sam","Treat-1-sorted.sam","Treat-2-sorted.sam","Treat-3-sorted.sam")

    fc=featureCounts(files,annot.ext="expressed-lnc.gtf",isGTFAnnotationFile=TRUE, GTF.attrType="transcript_id", allowMultiOverlap=TRUE, strandSpecific=1, isPairedEnd=TRUE)

write.table(x=data.frame(fc$annotation[,c("GeneID","Length")],fc$counts,stringsAsFactors=FALSE),file="counts.txt",quote=FALSE,sep="t",row.names=FALSE)

The counts.txt output looks like this:

GeneID  Length  Con.1.sorted.sam    Con.2.sorted.sam    Con.3.sorted.sam    Treat.1.sorted.sam  Treat.2.sorted.sam  Treat.3.sorted.sam
TCONS_00010754  676 0   0   0   0   0   0
TCONS_00003631  1769    0   0   0   0   0   0
TCONS_00009207  659 0   0   0   0   0   0
TCONS_00000101  283 15  31  17  12  12  9
TCONS_00005923  1184    0   0   0   0   0   0
TCONS_00007573  1205    0   0   0   0   0   0
TCONS_00009271  761 0   0   0   0   0   0
TCONS_00000248  590 0   0   0   0   0   0
TCONS_00000249  864 0   0   0   0   0   0
TCONS_00003762  680 0   0   0   0   0   0
TCONS_00000286  848 0   0   0   0   0   0
TCONS_00000293  533 0   0   0   0   0   0

Am I missing something? Previously I didn't use allowMultiOverlap=TRUE but saw somewhere that this might give actual counts for the genes and it worked for some people. However it didn't work for me. I am new to this field so I don't have much knowledge regarding bioinformatic tools and commands. Any help would be appreciated. Thank you.