Stags Head Sumach

I have a 2 sumach in my garden; one large and one small from a sucker. The larger tree has been very late developing leaves this year. Late June and still only small buds. The smaller tree is much further on. Any ideas as to why this might be the case.
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If there are no obvious signs of fungal infection , perhaps all it needs is a real good 'thinning-out' ; this should encourage larger leaves on the remaining branches. Rhus typhinus can be cut to ground if need be .