I have prepared 4 libraries (Nextera exome 37mb), first with 12 samples and prepared 3 months before the next 3 (of 24 samples). The first had low enrichment (40% of reads on-target), the second batch of 3 (dropped to 15% of reads on-target). The Hyb buffer, bought 2 months before the first batch, expired one week before the batch of 3... Is that really the reason for the drop in hyb efficiency? It didn't go through a thaw until I used it, I really cannot understand why that would be problem. Also considered the PCR block for the hyb, but another person also preparing libraries, used it and was unaffected. Any ideas?