SVT - Benadryl is an anti-histamine. Benadryl slows desensitization of Beta receptors (i.e. Upgrades them) by inhibiting phospholipase A2, which is the enzyme that breaks down methylated phospholipids, and this action in turn keeps the phospholipid membrane stable, and thus keeps the receptors functioning properly. This will allow you to use clen for much longer and it'll still have the same effects. Also, since Benadryl is an anti-histamine, and histamines have a direct effect on beta-adrenoreceptors (not just Beta-2’s but all of them), using an anti-histamine will have a direct effect on reducing beta-receptor stimulation, and thus upregulating your beta-receptors. Nytol can also be used for similar effects. I have used the Nytol 'One-A-Night' which has 50mg of the active ingredient 'Diphenhydramine Hydrochloride'. You should begin to take this after 18 days of clen (as this is when Clen starts to lose its anti-catabolic effect) and continue to take for 7-10 days. After cessation of Diphenhydramine Hydrochloride you should be fine for another 18 days without having to use it again. 50-100mg per night (just before you go to bed as they make you drowsy).
From personal experience I think you may not be taking enough Clen. I used to do my fair share of drugs that have an affect on the adrenal gland (i'm sure you can guess what these are) when I was younger and have built up a kind of tolerance to stimulants. I am currently on 200mcg ed clen and 150mcg t3 with amazing results! LBM has remained constant for the last 3 weeks!
If you have benadryl/ketotifen/nytol at hand you could add an ECA stack if you felt you still weren't getting results although I wouldn't advise it.
What is your diet like? To get best results from Clen/T3 you should be aiming for 40/20/40 (protein/carbs/fats). Also consider doing at least 45 mins of moderate intensity cardio first thing in the morning before food.
Hope some of this helps
p.s. I'm doing a six week cycle straight through