Please be aware that when we manufacture leaded sealed units we have to cut back the inner surface of the lead to allow for the spacer bar to sit flush on the glass surface.
We have encountered issues recently where our customers expect the inner lead to be touching the spacer bar, leaving no gap. However, if the lead is tight up against the spacer bar it can lift off the surface of the glass under expansion and consequently lose its adhesion properties. Therefore, it is important that there is a small gap to allow for the expansion and contraction of the lead and this is quite normal.
Please remember that this is a bespoke, hand-made product and, whilst we will endeavour to ensure that the gap is consistent throughout the unit, small differences in the size of the gap are to be expected.